The electrochemical performance of spinel Li4Ti5O12 (LTO) nanoparticles synthesized by flame spray pyrolysis with an average diameter of approximately 20–30 nm is reported in this manuscript and compared with that of micro-sized LTO particles (1–2 μm) formed by a thermal post-treatment of the nanoparticles. The significantly advanced high rate capability of nano-sized LTO is evidenced by the results from the galvanostatic tests with applied current densities of up to 17.5 A g−1, corresponding to a full (dis-)charge of the cell within less than 40 s. For nano-sized LTO, specific capacities of 115 and 70 mAh g−1 were obtained for applied rates of 10 C and 100 C, respectively, thus confirming the essential influence of particle size of lithium titanate on its high rate capability and practical power density. Moreover, a capacity retention of around 94.8% was observed after 1000 cycles at 10 C, presenting LTO nanoparticles synthesized by FSP as highly promising anode material for high power lithium-ion battery applications.