Abstract: Terminal mobility in heterogeneous networks is a key factor for wireless resource allocating. The paper presents an angle mobility model for the terminal, and analyzes the characteristic of the direction angle for terminal mobility in this model. According to the characteristic of the direction angle for terminal mobility, the theoretical closed-formula is derived for vertical handoff rate in the condition that users’ moving direction is unchanging in the fixed time interval T. And the theoretical ultimate value of handoff rate is taken as a special case when the fixed time interval T tends to zero. It can be clearly demonstrated that the relationship between handoff rate and the users’ distributed density, moving speed, cell radius from the theoretical ultimate value of handoff rate. The calculation result of handoff rate illustrates that the number of cell’s users is dynamic equilibrium under certain conditions. Then it further deduces the closed-formula of vertical handoff rate in heterogeneous networks, and reveals the validity of theoretical analysis by simulation. The simulation results show that the theoretical ultimate value of handoff rate can be used to approximate to the practical handoff rate with terminal’s frequently changing moving direction, low moving speed or small enough cell radius, thus it can effectively reduce the complexity of theoretical calculation for handoff rate.