Antimony trioxide (Sb2O3) nanoparticles with particle sizes ranging from 2 to 12 nm, spherical in shape, and well-distributed were successfully synthesized by chemical reduction method. The nanoparticles were synthesized in the presence of hydrazine as a reduction agent in ethylene glycol through the reaction between antimony trichloride and sodium hydroxide. Effects of reaction temperature, reaction time, precursor concentration and boiling temperature on the particle size, shape, and distribution of the Sb2O3 nanoparticles were investigated. Morphology of the nanoparticles was examined by transmission electron microscope (TEM). It was revealed that the particle size increased when reaction temperature, reaction time and concentration of precursor were increased. Moreover, the mixture needed to be boiled prior to the addition of hydrazine as a reduction agent, in order to obtain an optimum reduction. X-ray diffraction (XRD) was employed to study the crystallinity and phase of the nanoparticles. The nanoparticles were determined as cubic phase of Sb2O3 (ICDD file no. 00-043-1071) by XRD. Interrelation between UV-vis absorption spectra of the nanoparticles and their particle size were obtained.