LiFePO4/C nanoparticles were synthesized by solid state reaction and controlled cooling rates. LiFePO4/C samples showed olivine phase irrespective of the cooling rate. However, the average particle sizes significantly changed with the cooling rate; the particle size was 200 nm when prepared by slow cooling (10°C/min) and 120 nm by fast cooling (air quenching). Assembled coin cells exhibited first discharge capacities of 143.7 mAh/g and 159.5 mAh/g at C/10 respectively for the slow cooled and fast cooled samples; fast cooling enhanced the specific capacity. However, at high 4C-rate the specific capacity reduced to 42 mAh/g and 81 mAh/g for both slow and fast cooled samples. In short, cooling rates during the synthesis of LiFePO4/C cathode material can be used to tailor the battery performance.