A stick-slip dynamic model of a two-gear set is proposed. In this model, time-varying mesh stiffness, tooth separation, friction between the gear teeth surfaces and potential stick-slip are considered. Considering the contributions of the vibration velocity to the gear teeth sliding velocity, transition conditions between the stick state and slip state are revealed. Dynamic analysis shows that, when stick appears, the vibration amplitudes are dramatically increased, and tooth separation occurs. Additionally, stick also induces half-frequency components of the mesh frequency and causes chaos. Parametric studies show that heavy loads and rough tooth surfaces increase the probability of stick and increase the time of the stick state over a single mesh period. However, due to the complex nonlinearity, the existence of a certain relationship between the gear rotation speed and the stick time is not confirmed.