An in situ hydrothermal synthesis process was explored to prepare nano-sized high performance lithium iron phosphate carbon composite (LFPin/C) as active cathode material for lithium ion batteries. Tartaric acid (TA), as chelating agent and carbon source, was added into glycerol/water solution forming a homogeneous precursor. The mixture was transferred to a hydrothermal reactor to take full advantage of the synergistic interaction between both organic compounds in the synthesis of LFPin/C. For comparison, the properties of the ex-situ synthesized LFPex/C composite, obtained by addition of TA after the hydrothermal step, were evaluated. Results of comparative experiments show that the in situ method is capable to improve the homogeneity and dispersity of the residual carbon in combination with calcination at 600 °C for 3 h. Cyclic voltammetry and electrochemical impedance spectroscopy showed that the in situ generated carbon matrix with embedded LiFePO4 particles can reduce the charge transfer resistance. LFPin/C features a large specific surface area of 22.3 m2 g−1 and uniform particle size distribution with a typical size in the range of 20–40 nm. In line with these advantages, the composite yields excellent cycle stability with initial discharge capacities of 166.1, 140 and 104 mAh g−1 at rates of 0.1 C, 2 C and 10 C, respectively.