Cadmium has been found to produce wide ranges of biochemical and physiological dysfunctions in humans and animals. In the present study, toxic effects of Cadmium nanoparticle (CdNP) in the gills of mud crab Scylla olivacea was evaluated. CdNP (20ppm ppm/kg) induced abnormal structural changes in the gills. In the case of male, gill lamellae showed increased haemal space, destruction of Pillar cells, epithelium dilation and reduction of the thickness of the gill lamellae. In the case of female, complete destruction of haemal canal and haemal cells were observed. CdNP also induced antioxidant enzymes such as Catalase (CAT), Glutathione Peroxidase (GPx) and Superoxide dismutase (SOD) in both male and female crabs. Whereas, in the case of mitochondrial enzymes only Lactate dehydrogenase (LDH) showed increased activity compared to Succinate dehydrogenase (SDH) and Malate dehydrogenase (MDH) which were recorded decreased activity upon CdNP exposure.