In this paper, mathematical characteristics of uplink interference region in device-to-device (D2D) communications underlaying cellular networks are analyzed. We consider that one CUE and one pair of D2D user equipments (DUEs) coexist. The DUE pair consists of a D2D transmitting user equipment (DTUE) and a D2D receiving user equipment (DRUE). Therefore, additional interference between the CUE and the DUE is introduced. First, the uplink interference region is defined and described. Next, mathematical characteristics of the uplink interference region are derived. Finally, blocking probability is formulated based on the mathematical characteristics values. The analysis and simulation results show that theoretical values of the blocking probability well match simulation results. This result can provide a theoretical foundation for resource allocation and optimization of D2D communications.