ZnO nanoparticles (NPs) have synthesized by using the co-precipitation method at two different reaction temperatures (50 °C and 70 °C) by using zinc nitrate hexahydrate and sodium hydroxide as precursors in an aqueous medium. X-ray diffraction, SEM, EDS, and FTIR analysis confirmed the formation of good quality ZnO NPs with some minor impurities. Here, Debye-Scherrer (DS) and size-strain plot (SSP) theoretical methods have been employed to inspect the crystalline size and the intrinsic strain from the XRD peak broadening analysis and obtained average crystalline sizes were found to be 24 nm and 29 nm for the NPs prepared at 50 °C and 70 °C, respectively. Further, ZnO NPs have shown broad absorption, narrower emission spectrum and wide energy bandgap; these characteristics help in the potential applications of optoelectronic devices and biosensors e.g. solar cell and fluorescence resonance energy transfer techniques.