AB blood group is determined by A and B genes located on each chromosome which inherited from parents. But unusual inheritance of A and B genes on the same chromosome has been reported as cis AB. Recently the authors have experienced one case of patients with cis AB blood type undergoing emergency adhesiolysis of small bowel obstruction. We transfused the patient with Rh A+ packed red blood cell and fresh frozen plasma with atypical delayed hemolytic transfusion reaction.