Over the past several years, sea trade have increased traffic by ships which highlighted a problem of unwanted species invading the surrounding seas through ship’s ballast water discharge. Maritime trade volume has continuously increased worldwide and the problem still exists. A specific case is the influx of European zebra mussels in the Great Lakes between Canada and the United States of America. These species are responsible for the underwater pollution of structures and waterways. As a result, the respective countries are spending billions of dollars in an effort to clean up the contamination and prevent pollution. As part of an effort to solve marine environmental problem, BWM(Ballast Water Management) Convention was adopted at a diplomatic conference on Feb. 13 2004. In order to comply harmoniously this convention by each country, IMO has enacted 15 Guidelines to uniformly apply internationally. This convention will be effective after 12 months from the date which 30 countries ratified accounting for more than 35% of the world merchant shipping volume. On Sep. 8 2016, Finland ratified this convention and effective condition was satisfied as 52 states and World Merchant Vessel Fleet 35.1441%. Thus, after Sep. 8 2017, all existing vessels shall be equipped with BWTS(Ballast Water Treatment System) in accordance with D-2 Regulation, which physically handles ballast water from ballast water exchange system(D-1 Regulation). They have been developed since 2004 which is adopted from this Convention. UV type was given basic approval from IMO for the first time in 2006. Since then, the number of them developments has continued to increase and in 2010 most of them have received basic and final approval from IMO. Up until now, about 70 Systems have received type approval. The number of vessels to be equipped with BWTS worldwide is about 69,000 as of 2015. Thus various ships and Systems have been developed around the world. To retrofit BWTS on existing ships basic regulations such as Classification Regulations, Flag Regulations and Port Regulations were applied to Retrofit Design. However, when considering the purpose of the design is to protect safety of crew and ship, it is questioned as to whether these criteria can only achieve a such purposes. In this study, we analyzed in detail the optimal design method using the Risk Analysis and Evaluation technique which is mainly used in the manufacturing factory or the risky work site comparing with the traditional design concept method applying various criteria. The Risk Assessment Method is a series of processes for finding the Risk Factors in the design process, analyzing a probability of the accident and size of the accident and then quantifying the Risk Incidence and finally taking measures. In this study, this method was carried out for three(3) treatment type such as Ultra Violet, Electrolysis and Ozone Gas injection Type on DWT 180K Bulk Carrier using “HAZOP(Hazard and Operability) Study” method among various Methods. In the UV Type, 113 hazardous elements were identified. There were 49 points as "Negligible(Acceptable)" Risk Level, 64 points as "ALARP" Risk Level. But “Unacceptable" Risk Level were nothing. It is shown Appendix A for a more detailed matrix of Risk Assessment of this Type. In the Electrolysis Type, 93 hazardous elements were identified. There were 20 points as "Negligible(Acceptable)" Risk Level, 73 points as "ALARP" Risk Level. But “Unacceptable" Risk Level were nothing. It is shown Appendix B for a more detailed matrix of Risk Assessment of this Type. In the Ozone Gas Type, 123 hazardous elements were identified. There were 75 points as "Negligible(Acceptable)" Risk Level, 48 points as "ALARP" Risk Level. But “Unacceptable" Risk Level were nothing. It is shown Appendix C for a more detailed matrix of Risk Assessment of this Type. In this study, Risk Assessment was used to identify Risk Factors for BWTS Retrofit respectively and it was found that the Risk Factors should be considered on the optimum retrofit design for crew’s safety and ship protection. To improve the identified these items for each treatment type on the Retrofit Design is additionally costed. However, these costs were independent of the number of identified hazards and the performance of three treatment type. The cost to improve the identified Risk Factors for each treatment type on the retrofit design is charged additionally comparing with original retrofit cost.