Ankle exercises are useful for preventing deep vein thrombosis, as they increase venous blood flow velocity. The cause for the increased venous blood flow velocity during ankle exercises may be the skeletal-muscle pump, but the mechanism is not clearly understood. The purpose of this study was to investigate the effects of the dorsiflexion angle and gastrocnemius muscle contraction on venous blood flow velocity during ankle exercises and to investigate the mechanism of the increase in venous blood flow velocity. The blood flow velocity in the popliteal vein, ankle joint angle, and surface electromyographic activity of the gastrocnemius muscle were measured at rest and during ankle exercises in the prone position in young healthy volunteers. The significant increase in venous blood flow velocity was observed during dorsiflexion phase, max dorsiflexion and during planter flexion phase. The peak venous blood flow velocity was different in each subject and classified into four types. The correlations of venous blood velocity to ankle joint angle and with the surface electromyographic activity of the gastrocnemius muscle were not statistically significant. These findings suggest that venous blood flow velocity increases not only during plantar flexion and dorsiflexion.