Across sectional study of epidemiological risk factors and associated bacterial pathogens was conducted on 216 lactating dairy cows in jimma town from Oct. 2016 to April 2017 to determine the overall prevalence rate, associated bacterial pathogens and to assess effect of risk factors on prevalence of mastitis. Upon physical examination of udder and teats the prevalence of clinical mastitis was 2.3% at cow level and 0.96% at quarter level. Using the California mastitis test (CMT) for detection of sub clinical mastitis, the prevalence of sub clinical mastitis at cow level and quarter level was 60.65 and 38.4% respectively. The overall prevalence of mastitis was 62.96. Out of 323 (38.4%) CMT positive quarters with evidence of sub clinical mastitis, the quarter infection rate for the right rare quarter was highest (41.3%) followed by left rear (38.8%), right and left front (each 36.7%). Up on microbiological examination of milk samples for both clinical and subclinical quarters, out of a total of 331 cultured, 271(81.9%) yielded bacteria whereas 60(18.1%) yielded no bacteria. A total of 263 bacterial pathogens were isolated whereas 8 were mixed infections with more than two different colonies. The main bacterial pathogens isolated were coagulase negative staphylococci (CNS) (26.6 %), Staphylococcus aureus (24.7%), Escherichia coli (13.31%), Streptococcus species (9.13%), and Pseudomonas aeruginosa (8.8%). Other bacteria isolated with low isolation rate were micrococcus species (3.04%), Corynebacterium species (3.4%), Actinomyces pyogenes (3.8%), Bacillus species (3.42%) and other gram negative rods (3.8%). Host factors such as breed, age parity and lactation stage had significant effect on the prevalence of mastitis (p‹0.05). Managemental and environmental factors such as bedding, ventilation, frequency of barn cleaning, udder washing and use of communal towel had significant effect on the occurrence of mastitis (p‹0.05). Whereas milking practice, drainage had insignificant effect on the prevalence of mastitis (p›0.05).