Two experiments were conducted, using a total of 224 (112 barrows + 112 gilts) crossbred pigs, to determine the effect of dietary ideal amino acid ratios, and supplemental carbohydrase in hulless barley (HB) diets, on pig performance and N excretion in manure. The four experimental grower and finisher diets were 1) a covered-barley (CB) control diet, 2) a HB (cv. Condor) diet, 3) same as diet 2 except that the soybean meal was replaced with supplemental lysine, threonine, methionine and tryptophan, to provide ideal amino acid ratios of 0.65 for threonine to lysine and 0.30 for methionine to lysine, and 4) same as diet 3 with supplemental carbohydrase (Ronozyme W) enzyme at 350 g t-1. All diets were fed ad libitum as pellets with free access to drinking water. The average daily gain (ADG) was not different (P > 0.10) between diets 1 and 2, or among diets 2, 3 and 4 during the grower or finisher periods. The feed conversion efficiency was higher (P < 0.01) for diets 3 and 4 than for diet 2 during the grower period, but it was higher (P < 0.05) only for diet 4 than diet 2 during the finisher and combined grower-finisher periods. Backfat thickness, carcass index and dressing % were not different (P > 0.10) among the diets. The absorption of dry matter (88.9 vs. 83.7%) and energy (87.8 vs. 84.1%) was greater (P < 0.05) for diet 2 than diet 1. The excretion of faecal dry matter was 31.6% lower (P < 0.01) for diet 2 than diet 1 during the finisher period. The N excretion was 22.0% lower in pigs fed diet 3 than diet 2. Ammonia, carbon dioxide and hydrogen sulphide concentration in stored manure were not different (P > 0.10) among the diets, but ammonia production was positively correlate with urinary N excretion. These results indicated that feeding HB diets with supplemental amino acids minimises the excretion of both faecal dry matter and N with no adverse effect on the performance of lean-genotype pigs.