Different sized tetragonal tin oxide nanoparticles (SnO NPs) were synthesized using Piper nigrum seed extract at three different calcination temperatures (300, 500, 900°C) and these nanoparticles (NPs) were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), dynamic light scattering (DLS) and Fourier transform infrared spectrophotometry (FT-IR). The optical properties were studied using UV-Vis and photoluminescence (PL) spectrophotometers. The generation of reactive oxygen species (ROS) was monitored by using a fluorescence spectrophotometer and fluorescence microscope. The cytotoxicity of the synthesized SnO NPs was checked against the colorectal (HCT116) and lung (A549) cancer cell lines and the study results show that SnO NPs were toxic against cancer cell lines depending on their size and dose. IC values of SnO NPs having average particle sizes of 8.85±3.5, 12.76±3.9 and 29.29±10.9nm are 165, 174 and 208μgL against HCT116, while these values are 135, 157 and 187μgL against A549 carcinoma cell lines, respectively. The generated ROS were responsible for the cytotoxicity of SnO NPs to the studied cancer cells and smaller size NPs generated more ROS and hence showed higher cytotoxicity over larger size NPs. The results of this study suggest that the synthesized stable nanoparticles could be a potent therapeutic agent towards cancerous cell lines.