Abstract: A total of 240 Cobb 500 broiler chicks, 18 days old, were allocated to 10 treatments groups, each of which included 4 replicates. Experimental treatments consisted of a 5×2 factorial arrangement with 5 levels of L-threonine (Thr) supplementation and 2 levels of stocking density (11.90 birds m-2 as the normal stocking density or 16.66 birds m-2 as the high stocking density). L-Thr was added to a basal diet at 0.0 (control group), 0.25, 0.50, 0.75 and 1.00 g kg-1 diet. Dietary Thr values were 0.69, 0.71 (deficient), 0.74 (adequate; 100% NRC), 0.76 and 0. 79% (excess) of the current NRC recommendations. At 42 days of age, dietary Thr levels had no significant effect on performance, immunity, plasma total protein and glucose (GLU). However, plasma cholesterol (CHO) levels decreased significantly (p<0.05) with L-Thr supplementation and the lowest value occurred at 0.74% total dietary Thr. In addition, plasma triiodothyronine (T3) and thyroxine (T4) levels were higher (p<0.05) at 0.76% total dietary Thr compared with the control group. The normal stocking density resulted in better performance (p<0.05) compared with the high stocking density. However, stocking density did not affect plasma total protein, total Ig, IgG, IgM, total lipids, GLU, CHO, T3 and T4 levels. Interactions between Thr level and stocking density were observed for plasma levels of total lipids and cholesterol. In conclusion, total dietary Thr level at 0.69% (93% of NRC recommendations) is sufficient to improve broiler performance or immunity under high stocking density. However, 0.74% total dietary Thr level has a positive effect on decreasing plasma total lipids and cholesterol levels during growing-finishing phase.