During the last decade, advancement of nanotechnology has revolutionized various fields such as electronics, optics, materials science as well as architecture, and medicine. However, their health and environmental impact is not fully understood. TiO2 and ZnO nanoparticles (NPs), which are abundantly used for commercial purposes, pose a great risk to environment. In this context, we examined the adverse effects of TiO2 and ZnO NPs of < 25 nm and < 100 nm sizes to nematode Caenorhabditis elegans. < 25 nm TiO2 and ZnO NPs showed LC50 of 77 mg/L and 0.32 mg/L respectively, while < 100 nm TiO2 NPs were non-toxic and LC50 of 2 mg/L was obtained for < 100 nm ZnO NPs. Our studies indicate that in both cases, smaller particle sizes are more toxic than larger NPs and ZnO NPs are more toxic than TiO2 NPs. Further, we observed low LC50 values for ZnO NPs, probably reflecting the sensitivity of C. elegans as a model for eco-toxicity studies of NPs.