Structural change of Cr-doped LiFePO4/C during cycling is investigated using conventional and synchrotron-based in situ X-ray diffraction techniques. The carbon-coated and Cr-doped LiFePO4 particles are synthesized by a mechanochemical process followed by a one-step heat treatment. The LiFe0.97Cr0.03PO4/C has shown excellent rate performance, delivering the discharge capacity up to 120mAh/g at 10C rate. The results suggest that the improvement of the rate performance is attributed to the chromium doping, which facilitates the phase transformation between triphylite and heterosite during cycling, and conductivity improvement by carbon coating. Structural analysis using the synchrotron source also indicates that the doped Cr replaces Fe and/or Li sites in LiFePO4.