This study on the tidal stream power plants related to new and renewable energy is investigated by numerical analyses about the fluid dynamic performance of vertical axis tidal stream turbines(VAT) with computational fluid dynamics(CFD). Based on the extensive research cases about VAT and their trends, it is understood that positive researches on VAT are necessary. This study performs the experimental and numerical analyses mainly on H-Darrieus turbines, which are easy to be produced and designed, among VAT. Numerical analyses analyzed the flow in unsteady flow conditions around two- and three-dimensional VAT using FLUENT. A rotating flow field is implemented for sliding mesh techniques for the rotation of rotor, and a turbulence model is used. The experimental analyses are carried out at the circulating water channel(CWC) of Korea Maritime University. The experimental equipments including the initial driving of turbines are installed in CWC considering the vibration of rotor shart induced by fluctuating load of rotor blade. The result of numerical analyses are compared with the experimental result. In case of the numerical analyses of a H-Darrieus turbine, the patterns of two- & three-dimensional analyses are similar, but there is some difference in the absolute values due to the various three-dimensional effects. In case of the numerical analyses, additional studies are needed as there is some difference based on the boundary conditions. The maximum efficiency of experimental results is approximately 36%, but it included a lot of uncertainty error. So additional studies are needed to explain the blockage effect of CWC. It is understood that based on the study results about VAT, the studies on various shapes of turbines like Davis turbine and Helical turbine are considered to be possible gradually.