Accurate estimation of postmortem interval (PMI) is one of the most important and difficult issues in forensic medicine. After death, the tissues undergo autolysis and biomacromolecules degrade. The relationship of extent of DNA damage and certain components of the oxidant/antioxidant balance in brain and femoral muscle tissues of rats killed by cervical dislocation or drowning, with the post-mortem intervals was investigated and examined by making a relation with the histopathological finding in brain tissue. The results showed that the oxidant/antioxidant balance in brain and femoral muscle tissue was shift insignificant favour of the oxidants at 24 hrs postmortem and lasted tell 96 hours post-mortem toward the oxidant. The percentage of DNA fragmentation was detected increased parallel to significant increase of oxidant level from zero to 96 hrs post-mortem. The histopathological alterations in brain and muscle tissues of drowned or cervical dislocation were found to be in full accordance with biochemical finding and DNA fragmentation. Our results suggest that biochemical analysis, DNA fragmentation as well as histopathological examination of brain and muscle tissues provide an accurate estimation of post-mortem intervals.