Numerical simulation has been carried out to analyze seawater flow characteristics for shroud geometry variations with tidal current generation turbine. Seawater flow field characteristics in the turbine system are largely influenced by the shroud geometry. The vortex flow area in the diffuser part of the shroud gradually grows with the increase of shroud angle, but its recirculation velocity gets smaller. The centerline velocity of shroud increases with the length of cylinder part when diffuser part length and angle are constant(2.5m, 0.733rad) for the cylinder-diffuser type shroud system. Seawater velocity on the tip of turbine blade is pretty high, and as radial distance increases from the turbine axis, there is more gap between the fluid velocity isolines. These results in this study will be applicable for optimal design of tidal current generation system.