Abstract—Investigation on Serotypes of Ovine Pasteurellosis and Evaluation the level of protective antibody titter before and after ovine pasteurollosis vaccination was conducted from May 2010 to April 2012 in model sheep breeding villages of Farta and Lay Gaint districts of Amhara Regional State, Northwest Ethiopia. The objectives of the study were to investigate the prevailing field serotypes of ovine pasteurollosis and to evaluate the level of antibody titer of ovine Pasteurellois after vaccination in model sheep breeding villages. Accordingly, blood from 71 sheep were collected aseptically using non-heparinized blood collecting tubes and examined for serotyping of Mannhaimia. haemolytica and P. multocida using indirect haemagglutination (IHA) test at National Veterinary Institute, Debre Zeit, Ethiopia. Accordingly, M. haemolytica serotypes were found with prevalence of A1 (33.1%), A7 (31.8%) and A2 (28.5%) respectively. The least prevalent sero-type was Pasteurella Multocida biotype-A (6.6%). In addition, sera from 40 sheep were aseptically collected and examined to evaluate level of protective antibody titer before and after P. multocida Bio-type A vaccination. The overall population protective antibody titer (> 1:16) before and after vaccination was 32.5% and 87.5% respectively. There was no significant difference (p > 0.05) in antibody titer detected between study location, breed and age. However, there was a significant difference (C² =25.2; df=1, p=0.000) in level of protective antibody titer before and after vaccination. In conclusion, Manhaemia haemolytica serotypes were highly prevalent in the study areas. Although Pasteurella multocida Bio-type A serotype was the least prevalent, the vaccine applied against it in the field was found effective in developing protective antibody in the vaccinated population. However, the monovalent killed P. multocida biotype A-vaccine may not protect the different Manhaemia haemolytica serotypes isolated in the study areas. Therefore, the development of a multivalent vaccine using the most prevalent Pasteurella serotypes will help to effectively prevent ovine pasteurellosis.