Reactive oxygen species were thought to take part in oncogenesis and cellular differentiation. The study was conducted to know about the ROS levels in dog tumors. The concentration of TBARS, SOD, GSH and GST were estimated in tumor tissues from 68 dogs after the surgical excision. Tumor tissues showed higher concentration TBARS, SOD, GSH and GST as compared to control tissues. Relative concentrations of ROS were also higher in the malignant tumor compared benign tumors. Tissue concentrations of ROS can be used as a marker to detect the presence and grading of tumors.