In this manuscript we have prepared nickel oxide nanoparticles (NiO NPs) using Calotropis gigantea latex (CGL) through green synthesis method in which the latex acts as a capping agent. The synthesis part was optimized using RSM-BBD methodology. Further the synthesized NiO NPs were subjected to various analytical techniques like UV–Visible spectroscopy, Transmission Electron Microscope (TEM), X-ray diffractometer (XRD), Energy Dispersive Analysis (EDX), Particle Size Histogram and Zeta potential. We observed the cuboidal shape of NiO NPs with 28 ± 2 nm as average particle size. Further the synthesized NiO NPs were subjected to degrade toxic diazo dye Congo red (CR) and organic compound 4-Nitro phenol in the presence of sunlight within short duration of time. The results indicate that the NiO NPs degrades 95% of CR and 94% of nitro phenol. © 2019 Elsevier B.V.