A novel route was designed to synthesize LiFePO4/C composites by using the Fe1.5P byproduct, mixed lithium salts, and permeated oxygen from air via a rheological phase method. The reaction process was investigated with various techniques. When the calcining time was increased from 10 to 30 h, the gradual formation of olivine structure was observed. The growth kinetics of the crystals was analyzed. SEM and TEM results indicated the as-synthesized LiFePO4 was constituted of small spheres covered with carbon particles. The discharge capacity of the LiFePO4/C composite prepared at ~700 ℃ for ~25 h could reach 139.7 mAh g^(-1) and still remained 130.2 mAh g^(-1) after 15 cycles at 0.2 C rate, comparable to that of the reported LiFePO4/C composite using conventional methods. Cyclic voltammogram confirmed the LiFePO4/C composite had a high purity and good lithium ion insertion/desertion redox behavior.