This investigation was carried out on a total of 770 dairy cattle from different localities and reared under various hygienic environments from July 2011 to   August, 2012.
Application of California mastitis test and bacteriological examination,  revealed that 291(37.8%) animals were positive; 11.2% and 26.6% for clinical and subclinical mastitis respectively. There was a significant difference (P≤0.05) on the prevalence of mastitis between those animals in different parity number .The prevalence rate of subclinical mastitis rose with an increase in parity number, while the prevalence rate of clinical mastitis decrease. The prevalence rate of clinical and subclinical mastitis was also significantly affected by lactation stages (P≤0.05), season (P≤0.05) .Similarly there was a significant difference (P≤0.05) on the prevalence rate of mastitis between animals kept under different hygiene of milking process. Bacteriologically staphylococcus, streptococcus, and E.coli were the predominant frequently isolates in this study ; 45.9% , 38.4% and 14.8% respectively. In Conclusion, good hygiene in  milking process, milking clinically infected animals at last , culling chronic mastitic carriers, treating clinically infected animals and dry cow period therapy could reduced the prevalence rate of mastitis .