An  experiment, with a factorial arrangement of treatments (3×3), was conducted to evaluate the response or Ross broiler chicks to three stocking densities and three levels of probiotic added in drinking water. Two basal diets were formulated and fed to chicks during the starter and finisher periods. A total of 540 chicks was randomly divided into nine experimental groups, kept in floor pens at three stocking densities (10, 12 or 14 birds/m²) and given probiotic (0.0, 1.0, or 2.0 ml/liter of water). Each group had three equal replications. All birds had free access to feed and water, and managed similarly. Growth performance, economic efficiency, carcass traits and blood parameters were determined. Increasing stocking density from 10 to 14 birds/m² during the entire experimental period led to significant increases in feed intake, mortality rate, and plasma total lipids, triglycerides and cholesterol but negatively affected feed conversion ratio, body weight gain, and concentrations of total protein, albumin and globulin in blood plasma. But stocking density did not affect carcass traits of birds. Water-added probiotic positively affected feed intake, feed conversion ratio, body weight gain and mortality rate, and caused a significant increase in total protein, albumin and globulin but significantly reduced plasma levels of total lipids, triglycerides and cholesterol. Added probiotic, however, had no effect on carcass traits. When growth performance and economic aspect are taken into consideration, it can be concluded that a stocking density of 10 birds/m² proved to be the best level. Probiotic  addition produced further improvement in growth and economic efficiency of broiler chickens.