The objective of the present study was to evaluate the effect of E. coli on serum biochemistry and to determine the preventive effect of adding marjoram as growth promoters in 2 concentrations (0.5% &1%/kg ration) on serum biochemistry. This study was carried out on 150 one day- old chicks to evaluate the effect of dietary supplementation of 0.5 and 1% origanum majorana for 6 weeks on broilers and the possibility of facing the experimental infection with E. coli O78. In treated groups with marjoram 1%, biochemical results revealed increase in the total protein and total globulin and decrease in AST, ALT and uric acid in 4^th and 6^th weeks of the experiment. In addition to significant decrease in serum cholesterol, triglyceride, LDL and increase in HDL, while, serum albumin showed non-significant changes compared to control group. Regarding to the infected group and group infected and treated with 0.5% marjoram, increased total protein, total globulin, AST, ALT, glucose, cholesterol and uric acid were observed