Lithium iron phosphate–carbon (LiFePO4–C) composites with various amounts of Fe2P are synthesized by ball-milling coupled with microwave heating to serve as cathodes for lithium-ion batteries. LiFePO4–C in which Fe2P is restrained below a critical concentration gives as very high discharge capacity of 165mAhg−1, excellent rate capability (85.4% C/50-rate discharge capacity at 2C) and stable cyclic retention for 250 cycles. Above the critical concentration however, electrochemical performance deterioated. Analysis and debate, based on a comparison of the physical and electrochemical properties among LiFePO4–C composites with the variation of Fe2P, proceeded to the conclusion that below the critical concentration, Fe2P enhanced the conductivity of LiFePO4–C, whereas above the critical concentration, it blocked the one-dimensional Li+ pathways in LiFePO4 and might hinder Li+ movement in LiFePO4. Therefore, in order to obtain a LiFePO4–C composite that enhances electrochemical performance, it is concluded that the amount of Fe2P should be carefully controlled below its critical concentration.