NiO-Ce0.8Sm0.2O1.9 (SDC) composite nanopowders, which are being investigated as a promising anode material for intermediate-low temperature solid oxide fuel cells (SOFCs), were successfully synthesized by hydroxide co-precipitation method. The effects of calcination temperature and time on crystal phase, crystallite size, average particle size and particle size distribution of the synthesized powders were investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis showed that the synthesized powders consisted of two phases, NiO and SDC, without any other noticeable phase. The NiO and SDC phases were formed after calcining at 500°C. The calcination temperature is the most important factor which affects the crystal phase, particle size and particle size distribution. With an increase in the calcination temperature, the average particle size of the synthesized powders increased and the particle size distribution became wider. The average particle size of the powders calcined at 600-900°C was in the range of 9.8-39 nm according to the TEM analysis.