Abstract: To create a mathematical model for the hydro-ballistic trajectory of high speed pellet entering water, the hydro-ballistic trajectory and cavity of sphere and pistol pellets entering water at three oblique angles and six velocities have been experimentally studied using a high-speed digital video recorder. The experimental results show that the shape of the pellet has significent influence on the hydro-trajectory stability. The hydro-ballistic trajectory of the sphere pellet obliquely entering water is more stable than that of the general pistol pellet. The initial cavity, ballistic trajectory and velocity attenuation of the sphere pellets are similar at different water-entry angles and initial velocities. The velocity of pellet in water decreases very quickly, and the attenuation is similar for both of the sphere and pistol pellets. A calculation model is proposed to predict the velocity attenuation of the pellet. Calculated results are in good agreement with the experimental.