ZnO nanoparticles were prepared by direct thermal decomposition of the precursor [contain: Zn4(SO4)(OH)6.H2O and ZnO] in air for 1 h at 875°C. The pH of the precursor solution was set at 6 and 11 by the controlled addition of the NH3•H2O solution. The as-prepared materials were characterized by X-ray diffraction (XRD), infrared spectrum (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). According to the analyses, the ZnO nanoparticles were pure with both rod-like and spherical shapes which were synthesized using chloride and sulfate solutions, respectively. Moreover, the average diameter of synthesized spherical ZnO at pH=6 was around 85±5 nm; while, an average diameter of the nanorods was 980 nm and 2.2 μm in length. The average nanorods diameter at pH=11 was 760 nm and 3.3 μm in length; while the average particle size of spherical shape was around 112±5 nm. The TEM and SEM image showed the morphology of spherical and nanorods particles. The reaction temperature of all steps during the synthesis of ZnO nanopowders shifted to high temperature, as the pH of the starting solution increased from 6 to 11. Due to the simplicity, the present method could be proposed as a convenient approach to produce pure ZnO nanoparticles by means of ZnSO4 and ZnCl2 solutions without using any toxic and organic chemicals.