For detection of crime in Forensic medicine blood group studies& its application plays an important role. The study of blood grouping gives important information regarding identification and details of Crime Investigation. The most common trace evidence is blood which is obtained from the crime scene. Blood grouping & typing can point to a group of people that have the same blood type rather than pointing particular person. When blood is properly collected and preserved, blood as an evidence can establish a strong link between a criminal and the criminal act. We can trust the blood grouping of blood stains in detection of crime. Randomly A, B, AB, Rh positive blood samples were taken in duplicates and rusted pins, dust particles, detergent, staphylococci are added to it respectively. The common contaminants in blood stains are dust, rust, bacteria and detergent. So an effect of these on blood groups was studied in detail.