Carbon coating is an effect way to improve the electronic conductivity of polyanion materials for lithium ion batteries. A citric acid assisted sol–gel method is employed in this study to prepare Li2MnSiO4/C composite, where carbon is in situ coated on Li2MnSiO4 during the synthetic process. The effect of in situ carbon coating on the structural, morphological and electrochemical characteristics is studied in detail. The particle size decreases whereas the amount of impurities increases with increasing amount of carbon. The Li2MnSiO4/C sample with 2.1wt% carbon shows the best electrochemical performance, and the particle of it is uniformly coated by a thin layer of carbon. The carbon coating is incomplete if the amount of carbon is lower than 2.1wt%. However, uneven thick layer and/or agglomerates of carbon are formed if the amount of carbon is higher than 2.1wt%. Li+ transfer on the surface of particles will be hindered if the carbon layer is too thick. Although the smaller particles of the samples with 8.5wt% and 10.5wt% carbon have a positive effect on their electrochemical performance, the combined negative effect of higher amount of impurities and thicker carbon layer with carbon agglomerates finally lead to poorer electrochemical performance.