A simple and effective method, the ultrasonic-assisted co-precipitation method, was employed to synthesize nano-sized LiFePO4/C. A glucose solution was used as the carbon source to produce in situ carbon to improve the conductivity of LiFePO4. Ultrasonic irradiation was adopted to control the size and homogenize the LiFePO4/C particles. The sample was characterized by X-ray powder diffraction, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). FE-SEM and TEM show that the as-prepared sample has a reduced particle size with a uniform size distribution, which is around 50nm. A uniform amorphous carbon layer with a thickness of about 4–6nm on the particle surface was observed, as shown in the HRTEM image. The electrochemical performance was demonstrated by the charge-discharge test and electrochemical impedance spectra measurements. The results indicate that the nano-sized LiFePO4/C presents enhanced discharge capacities (159, 147 and 135mAhg−1 at 0.1, 0.5 and 2C-rate, respectively) and stable cycling performance. This study offers a simple method to design and synthesis nano-sized cathode materials for lithium-ion batteries.