We report a simple solution-based method to synthesize phase- and size-controllable ZnS nanoparticles at low temperature. Cubic ZnS (c-ZnS) and hexagonal ZnS nanoparticles (h-ZnS) were obtained by heating an aqueous solution of Zn(NO3)(2)center dot 6H(2)O and Na2S2O3 center dot 5H(2)O at different temperatures. When the system was heated at 65 A degrees C for 24 h, hexagonal crystal structure of ZnS nanoparticles, with size of 50-350 nm, was obtained, as confirmed by X-ray diffraction and selected-area electron diffraction. When the reaction temperature was 100 A degrees C under hydrothermal condition, c-ZnS nanoparticles were obtained and exhibited monodisperse nanoparticles with average size of 4 nm. Proper rate of S releasing tuned by the variation of pH value is believed to be critical to stabilize the hexagonal ZnS nanoparticles. Compared with large size of h-ZnS nanoparticles, c-ZnS nanoparticles show higher photocatalytic activity in degrading methyl orange (MO). The degradation efficiency of c-ZnS nanoparticles reaches 97% under UV irradiation for 120 min. The good ultraviolet absorbing ability, charge separation property, and large surface area of c-ZnS nanoparticles are believed to have a positive impact on improving the degradation rate and degradation efficiency of MO.