Introduction: ABO and Rh blood group systems are the most important blood grouping systems from clinical aspect. Determination of blood group is important for blood transfusion therapy, medico-legal purposes, organ transplantation, settlement of paternity disputes etc.Methods: A cross-sectional descriptive study was carried out for a period of one year from 1st January 2011 to 31st December 2011 in blood bank of Tribhuvan University Teaching Hospital. All blood samples collected for blood group determination were included in the study. Blood group was determined by slide agglutination method using commercial antisera.Results: A total of 13568 blood samples were analyzed, 5123 (37.75%) were male and 8445 (62.25%) were female. Frequencies of blood groups A, B, AB and O were found to be 4034 (29.7%), 3665 (27.0%), 1114 (8.2%) and 4755 (35.1%). Frequencies of Rh positive and Rh negative blood groups were found to be 13200 (97.3%) and 368 (2.7%). Blood group O was common in Brahmin, Chhetri, Tamang, Lama, Gurung, Sherpa, Terai Brahmin, Muslim and Yadav ethnicities; blood group A was common in Newar, Rai, Magar, Limbu and Sanyasi ethnicitites; and blood group B was common in Tharu and Marwari ethnicities.Conclusions: Blood group O was found to be the most common blood group while AB was the rarest one. It was found that blood group O is the more common in Sherpa, Brahmin and Yadav; A in Limbu, Rai and Newar; and B in Tharu and Marwari ethnicities._______________________________________________________________________________________Keywords: ABO; blood group; ethnic distribution.