OFDM (Orthogonal Frequency Division Multiplexing) has been widely used in modern wireless communications because of its high data rate, immunity to delay spread and frequency selective fading, high frequency spectral efficiency and other advantages. Besides these benefits, one of the main disadvantages is the PAPR problem. High PAPR will increase the system complexity and restrict the system performance. Within the existing PAPR reduction methods, SLM (Selected Mapping) is one of the most popular. The PAPR performance of SLM-OFDM is improved by efficient phase rotation factors, which will increase the size of side information and reduce the information data rate. In this paper, we propose a method for SLM to choose the most efficient phase rotation factors, so that SLM-OFDM can get efficient PAPR performances with limited size of side information. Through simple analysis, we compare the PAPR performances of different phase rotation factors and make a conclusion of the regulation.