Li2MnSiO4/C nanocomposites have been successfully prepared by a two-step method to accurately control the formation of the orthorhombic Pmnb phase, and phosphate substitution is used to improve the electrochemical performance of this cathode material. The structure and electrochemical properties are investigated using X-ray diffraction, scanning electron microscope and electrochemical tests. The crystal structure of this material is confirmed to be Pmnb polymorph and the average grain size is about 80–100nm. The electrochemical tests indicate that Li2.1Mn0.9[PO4]0.1[SiO4]0.9/C nanocomposites deliver a discharge specific capacity of 216.9mAh·g−1 after initial activation and it shows much enhanced electrochemical performance compared with Li2MnSiO4/C nanocomposites. Furthermore, the XRD patterns, HRTEM images and Raman analysis reveal the crystal structural changes of the two samples during cycling. Thus, this material has shown potential for high power applications in Li-ion batteries.