The lithiation/delithiation of phosphate electrode materials in lithium ion batteries is often accompanied by an electrochemically driven phase transformation. The good understanding of phase transformation benefits for evaluation of SOC (state of charge) by OCV (open circuit voltage). A facile GITT (Galvanostatic Intermittent Titration Technique) is attempted to investigate the phase transformation kinetics of multi-particles LiFePO4 (LFP) nanoparticles in different depth of charge/discharge. The process of charge/discharge is divided by 20 sections, in which the cell is charged/discharged by the 5% capacity at different C-rates and then gets rest for 30minutes. The polarization can include the kinetics of phase transformation and concentration diffusion. The detailed analysis of polarization data reflects the information of phase transformation, especially kinetics of phase transformation. Here, the result shows that the higher C-rates make quicker equilibrium of OCV in 30minutes rest. This is consistent with the phase transformation theory as proven in this paper that the higher C-rates make the faster phase transformation. It is also displays that phase transformation is different between two ends and middle parts, and dependent on C-rates. The proposed approach paves a facile and effective way to investigate the phase transformation of phosphate electrode.