This experiment was carried out at the Field of Poultry, Department of Animal Resources, College of Agriculture, University of Baghdad, during the period from 1/5/2011 until 5/7/2011 to study the effect of adding arginine to laying hens diet on certain blood traits. A total of 100 Brown Lohmann laying hen chickens, 38 weeks of age, were randomly distributed into four treatment groups, with 25 hens for each treatment. Treatment groups were: T1: bird-fed diet with no additional arginine (control group); T2, T3, and T4: bird-fed diet supplemented with 0.4, 0.7, and 0.9%, respectively. Therefore, the total amounts of arginine in the four treatments (T1, T2, T3, and T4) become 1.1, 1.5, 1.8, and 2.0%, respectively. Results of this experiment revealed that there were no significant differences (p > 0.05) between treatment groups regarding the total red blood cell count (RBC) as well as with respect to hemoglobin concentration (Hb) and packed cell volume (PCV). It was also noticed that there were no significant differences (p > 0.05) between treatment groups concerning each of mean cell volume (MCV), mean cell hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). Moreover, the results of this study also indicated that supplementing laying hens with different levels of arginine (T2, T3, and T4) resulted in a significant increase (p<0.05) in total white blood cell count (WBC) and percentage of heterophil (H) cells and a significant decrease (p<0.05) in lymphocyte (L) cells, monocyte cells, and H/L ratio as compared with the control group (T1). In conclusion, adding arginine to the diet of laying hens at levels higher than the levels recommended by the NRC did not have a negative effect on the physiological performance of birds, as indicated by the non-significant differences between treatment groups as regards RBC, PCV, Hb, MCV, MCH, and MCHC. However, supplementing arginine to the diet of laying hens resulted in an enhancement of the immune response, as indicated by a significant increase in WBC in comparison with the control group. On the other hand, adding arginine to laying hens diet didn’t cause any stress on birds, as indicated by the significant decrease in H/L ratio as compared with the control group.