The purpose of this study is to estimate undrained shear strength and preconsolidation pressure of marine clays using N-values obtained from standard penetration test(SPT). The study was executed on the marine clay located in the west coast of Korean Peninsula: Kyounggi area(Kyounggi bay), Chungcheong area(Pyeongtaek & Asan), Jeonbuk area(Kunsan and Janghang). The collected sample of marine clays were classified into CH, CL, and ML based on the unified soil classification system. The N values and SPTs and the physical and mechanical properties obtained from the laboratory tests were used to draw the following conclusions. The results of this study can be usefully used as soil design parameters for a planned construction project. In addition, it can be the soil properties of some areas where it is difficult to perform the tests in west coast area can be estimated using the results of this study.