In the present investigation of the human pathogenic bacteria such as Escherichia coli, and Pseudomonas fluorescens, Salmonella typhi, and Klebsiella pneumoniae, were analysed the ozone treatment by using ozonizer M221. To optical density was control and treated cultures were measured and to the different time intervals such as 5, 10 and 15 minutes of the optical density (by use of a digital spectrophotometer at 540nm) were gradually decreased. Among the treated bacterial species E.coli revealed high sensitivity to ozone treatment and compared to other bacterial strains. This was shown by the optical density value of Escherichia coli, and Salmonella typhi; Klebsiella pneumoniae and Pseudomonas fluorescens such as 0.81, 0.5.0.2 and 0.01; 0.290, 0.261, 0.245 and 0.221; 0.256, 0.254, 0.235 and 0.220; 0.219, 0.17, 0.08 and 0.045. The numbers of cells surviving after ozone treatments were less than for untreated cells.