This study was carried out on 51 Friesian cows out of 488 suffering from sub-clinical mastitis in 3 farms at Assiut Governorate (Bani-mor, El Awamer and Secondary Agricuture School). The obtained data showed that the total incidence of subclinical mastitis (S/C/M) in Assiut Governorate was 11.38%, while the incidence in 3 farms (Bani-mor, El Awamer and Secondary agriculture school was (7.1%, 29%, and 14.3% respectively. The number of the bacterial isolates in this investigation was 55: The number and frequency percentages of the isolates were Staphylococcus aureus 16 (29.1%), Staphylococcus epidermedis 10 (18.20%), E. coli 11 (20%), Corynebacterium bovis 6 (10.90%), Streptococcus agalactiae 7 (12.70%), Streptococcus dysagalactiae 3(5.46%) and Citrobacter 2 (3.64%) respectively. Also the sensitivity of the isolated organisms to eleven antibiotics were discussed. It was found that most of the isolated species was highly sensitive to Spectram, Chloramphenicol, Cephaloxine, Remectan and Gentamycine, moderate sensitive to Kanamycin and less sensitive to Tetracycline and Streptomycin but was resistant to penicillin and Amoxicillin.