The aim of this experiment was to examine the effect of different stocking densities on behavior, performance, and some blood parameters of Muscovy ducks. One hundred thirty-five-1-day old male Muscovy ducklings were divided in to 3 treatments with 5 replicates per treatment for a 60-day trial. Birds were housed at 6 birds per m2 (Control), 9 birds per m2 (Moderate stocking density) or 12 birds per m2 (High stocking density). The stocking density effects on duckling's behavior, performance, and some blood parameters were investigated. At the end of the experiment, production and blood parameters were examined. The results showed that, stocking density birds exhibited less feeding, crouching, standing, walking and feather ruffling but more drinking, huddling, sitting, preening, feather pecking and wall pecking behaviors compared control ducklings (P < 0.05). The stocking density ducklings also had lower body weight, feed intake, body weight gain, eviscerated weight, dressing % compared to control birds (P < 0.05). At day 60, the serum calcium, phosphorus, total protein, thyroxine (T4) and total antioxidant of stocking density birds were lower, however, malondialdehyde and corticosterone were higher than that of control ducklings (P < 0.05), T3 and total albumin showed significant decrease only in high stocking density compared to control one (P < 0.05). These findings propose that rearing Muscovy ducks at 9 or 12 birds per m2 during fattening phase led to harmful influence on duck behaviors, performance, carcass characteristics and physiological parameters, with the potential to rear six birds on one square meter.