The mechanism and the producing condition of instability in low velocity of hydraulic motors were analyzed based on dynamic model and test. Nonlinear coupled force combined with hydraulic spring and self-excited vibration was thought to be the key factor that could affect instability in low angular velocity. The results indicated that the low speed creeping characteristics of hydraulic motor could be affected by friction torque, leakage and compressibility of oil and liquid. The test showed that the fluctuation was the negative characteristics of friction in the self-excited vibration conditions. Some measures were proposed to improve the low speed stability of hydraulic motors.