The physical properties and electrochemical performance of two kinds of commercialized LiFePO4 (hereafter abbreviated as LFP) were analyzed. And the cycle stability of the two kinds of material in the 18,650 type lithium ion cells was measured at different charge cut-off voltage (3.65,3.8,4.0 and 4.2V). The two kinds of material showed remarkable difference. One has good cycle stability but another encounters serious failure problem at high charge cut-off voltage (4.2V). Results showed that the impurities containing iron atom in the raw material and formed during the charge–discharge cycles are the main reasons of the failure mechanism. The impurities were reduced on the graphite surface and iron dendrite formed gradually, which may penetrate through the separator and results in the inner short circuit of the cells.