Power-free underwater vehicle with two rudders is developed and works in Kuroshio. Applied loads and strength of the rudder are determined by CFD and ANSYS at 0.3~1.8 m/s. A tested vehicle was conducted at 0.65 m/s flow in harbor. We concentrate on attack angle and strength of rudders of power-free underwater vehicle in low flow velocity. Results indicate that only 73.7% effective downward pressure of the rudder was measured due to the influence of turbulent flow. Pitch angle applied to the vehicle is essential to assure definitely submersible. Front rudder and rear rudder had better not be aligned in order to reduce the effect of wake flow. At flow velocity of 1.8 m/s, maximum lift force and drag force on the rudder are -13.0 kN and 8.05 kN, respectively. Maximum displacements locate on both end of rudder with a value of 0.673 mm. Deformation ratio of the rudder is 1/3165. Numerical calculations and the experiments reveal the rudder of powerfree underwater vehicle is elastic.