In this paper, a new initial radius (IR) selection method of Sphere Decoding (SD), called IR-ZF-OSUC, for MIMO system is proposed. An significant merit is that this method utilizes the result ofQR decomposition which is inherent in SD to obtain a suboptimal solution, and selects the distance between the received signal and the lattice point mapped by the suboptimal solution as the IR, which makes the procedure can be embedded in the body of SD. Unlike other approaches, it needs no extra processing, so lower computational complexity is achieved. Additionally, this method includes an ordering step, which not only makes the IR be closer to the optimal value but also increases the efficiency ofSD itself. The simulation results show that the computational complexity ofSD with IR-ZF-OSUC is lower than the ones using other approaches over a wide range ofSNRs.