The Present study was designed  to investigate  the prevalence of Subclinical  mastitis.  A total  number  of  one  hundred  and  sixty (160) cow's  quarter  milk  samples  were  collected  from  different  dairy  farms at Sharkia Governorate  for detection  of the causative agents  of  Subclinical mastitis. The  results  revealed  that  California mastitis test  (CMT)  was graded as (- , ±  , + , ++ , +++ )  with  incidence  of  ( 12.5 , 6.25  , 34.375 , 25.526 , and 21.25 % ) respectively,  the  mean  ± SEM   of  milk electrical  conductivity  ( EC ) ( ms / cm ) of  6.5 ± 2.5  , and the  mean  ± SEM  of milk Somatic cell count ( SCC) ( cells / ml  ) was   547.5 × 10³ ± 507.5 × 10³  , the   mean  ± SEM  of  milk  Chloride  %  was  0.235 % ± 0.165 %    , the  mean  ± SEM  of  measured  Fat %  was   2.65 %  ±  1.15 %  , the   mean  ± SEM  of  measured  Protein  %  was   3.1 % ± 1.1 %  , the   mean  ± SEM  of  measured  Lactose  %  was 3.55 % ± 1.45 % , the   mean  ± SEM  of measured  SNF %  was 7.5 % ± 1.5 % . The  most  predominant  single  pathogens  in  100  out  of  160   milk  samples  was  S . aureus,  S.agalactiae,  S.dysagalactiae, S.uberis  and  E.coli with  incidence  of   (21.875,  15.625,  12.5 ,  6.25 and 6.25 % )  respectively,  and  normal  healthy  control  20 ( 12.5 % ) milk samples  (didn't  yield  any pathogens), and  40 ( 25 %)  milk  samples  yield  mixed  bacterial  pathogens . It  can be  concluded  that  CMT  was  used  to determine  the  severity  of  Subclinical  mastitis.  CMT  positive  and  SCC>250.000 (cells / ml ) in individual  quarter  foremilk  samples was found  to  be  accompanied  by several  production  effects  and  sever  depression  in  milk  chemical parameters. Bacterial contamination  of  milk  from  affected  cows  render it  unfit  for  human  consumption,  and  there  is  correlation  between  SCC  and  decrease  chemical  milk  parameters.