The study was carried out to investigate the effect of diazepam in drinking water on some productive and physiological traits of hybrid (Cobb500) broiler chickens. Forty birds (male and female) were used at age of three weeks to determine the suitable concentrations of diazepam per milliliter of drinking water which can be practically employed to calm down the birds , without producing too much sedation. Two levels of diazepam were chosen (0.02 and 0.04 mg/ml) from the four levels tested (0.01, 0.02, 0.04, 0.08 mg/ml). At four weeks of age, 90 birds (45 males and45 females) were chosen, relatively homogenous in body weights, and divided into three groups. The first group (the control) was given drinking water free of diazepam, the second and third groups were given 0.02 and 0.04 mg of diazepam / ml of water, respectively. Data indicated that there was an increase in the average body weights for the groups treated with diazepam at age of five, seven and eight weeks .Also an increase in the amount of feed consumed ,an improvement of feed efficiency ,and an increase in dressing percentage for groups treated with diazepam. No significant differences were found between any of the treated groups for the diazepam and the control groups, in the levels of glucose, cholesterol and triglycerides.