This experiment was conducted to determine the ileal digestibility of CP and AA in casein by regression analysis and to investigate the effects of casein in experimental diets on the apparent ileal digestibility (AID) and standardized ileal digestibility (SID) of CP and AA in corn distillers' dried grains with solubles (DDGS) fed to pigs. Twenty barrows (initial BW = 50.5 ± 4.46 kg) surgically fitted with T-cannulas at the distal ileum were used. Eighteen pigs were assigned to a triplicate 6 × 3 incomplete Latin Square design with 6 diets and 3 periods. Two pigs were used as replacements. Three diets were prepared to contain 60, 100, or 140 g/kg casein to determine the ileal digestibility of CP and AA in casein by regression analysis, and 2 diets were prepared to contain either 480 g/kg DDGS or 308 g/kg DDGS and 60 g/kg casein. Casein and DDGS were the sources of nitrogen (N) in experimental diets. A N-free diet was prepared based on cornstarch and dextrose. Experimental periods consisted of 5 d of adaptation and 2 d of ileal digesta collection periods. The AID of CP and AA in casein linearly increased (P < 0.01) with increasing concentration of casein in the diets. The SID of indispensable AA in casein ranged from 94.7% (SEM = 1.16) for Ile in pigs fed the diet containing 60 g/kg casein to 103.3% (SEM = 2.21) for Arg in pigs fed the diet containing 100 g/kg casein. Except for Arg, pigs fed the diet containing DDGS and casein had greater (P < 0.001) SID of CP and indispensable AA than those fed the diet containing DDGS without casein. The ileal digestibility of indispensable AA in casein determined by regression analysis ranged from 96.8% (SE = 3.14) for Thr to 103.1% (SE = 4.40) for Arg. The AID and SID of CP and AA for DDGS in the diet containing casein were calculated by difference method using the ileal digestibility of CP and AA in casein determined by regression analysis. The AID of Lys for DDGS in the diet containing casein was greater (P = 0.035) than those without casein; however, the AID of CP and the other indispensable AA, except for Arg, Phe, and Trp, for DDGS in the diet containing casein was less (P < 0.05) than those without casein. The SID of Lys and Phe for DDGS in the diet containing casein was greater (P < 0.05) than those without casein. In conclusion, improved AA composition in semi-purified experimental diets may affect the SID of AA in low-quality protein ingredients.