During January-November 2012, a total of 2,041 quarter milk samples were collected from dairy cattle of 82 dairy farms nationwide. About 42% (870/2,041) of the samples that had somatic cell counts (SCC) of ≥ 200,000 cells/ml were subjected to microbiological examination. No bacteria was isolated from 95 of 870 (10.9%) samples. Among 1,237 bacteria isolated from the rest 775 samples, 1,085 were identified with VITEK: more than half (52.1%, 645/1,237) of the isolates were gram negative bacillus. Gram positive cocci including Staphylococcus accounted for 35% of the isolates and almost none of gram positive bacilli isolated. Excluding Coagulase-negative Staphylococcus (CNS), the most frequently isolated bacterial species was Escherichia coli (11.2%, 138/1, 237), followed by Klebsiella pneumoniae (8.1%,100/1,237), Staphylococcus aureus (7.1%, 88/1, 237), Enterobacter cloacae (6.0%, 74/1, 237), and Serratia marcescens (3.5%, 43/1, 237). The most common resistance of S. aureus was to penicillin (77.4%) and ampicillin (73.0%), while no resistance was observed against gentamicin and cephalothin. Although CNS presented resistance to all antimicrobials tested but the most prevalent resistance was to penicillin (35.6%) and ampicillin (37.0%). The pattern of antimicrobial resistance observed in CNS was similar to that of S. aureus, but the rates were much lower than those of S. aureus. E. coli also showed resistance to all the antimicrobials tested, although the rates were not very high. The highest resistance of E. coli was to cephalothin (39.4%) and ampicillin (36.2%), while most of the strains (98.0%) showed sensitivity to amikacin. The results of this study provide information on current situation of bovine mastitis in Korea.