The handoff rate under base stations (BSs) cooperation for the user's two transitions in ultra-dense network (UDN) is analysed in this study. Frequent handoffs in UDN cause the degradation of user experience, which should be considered for UDN deployment. The determination of cooperation area to prevent the handoff of the first transition (from the origin to the first destination) is investigated and an approximate form that is convenient for practical application is given out. The handoff rate of the second transition (from the first destination to the second) is then studied with the BSs cooperation. Some special cases, where the mobility angle is set at some fixed values are also investigated. It is shown that the handoff rate is significantly affected by both the direction and the length of the user's transition. Moreover, the correlation between the two transitions does impact on the performance of handoff rate under the BSs cooperation. Numerical results are finally presented to verify the authors’ analysis.