Two experiments were conducted to evaluate the effects of diet complexity and fermented soy protein on growth performance and amino acid digestibility. In Exp. 1, a total of 120 crossbred weanling pigs ($5.68{pm}0.80;kg$ BW) were randomly allocated into 4 treatments. Each treatment had 6 replicate pens comprising 5 pigs in each replicate. Experimental diets consisted of simple (soybean meal as protein source) and complex (soybean meal, rice protein concentrate, potato protein concentrate and fish meal as protein sources) diets; each diet contained 0 or 5% fermented soy protein (FSP), respectively. Dietary treatments included: i) simple diet; ii) simple diet with 5% FSP; iii) complex diet; iv) complex diet with 5% FSP. Pigs were provided each experimental diet for 20 d (phase 1) and then fed the same common diet for 10 d (phase 2). During days 0-10, pigs fed FSP diets had greater ADG than those fed non-FSP diets (p<0.05). G/F in FSP treatments was significantly higher than that in non-FSP treatments (p<0.05) from days 0 to 10. Throughout the overall period, G/F was greater in FSP treatments compared with non-FSP treatments (p<0.05). On d 10, N digestibility was higher in pigs fed FSP diets than in those fed non-FSP diets (p<0.05). Diet complexity did not affect growth performance and nutrient digestibility (p>0.05) in this experiment. In Exp 2, 12 ileal-cannulated, weanling barrows were housed in individual metabolism crates and randomly assigned to 1 of 4 treatments (same as Exp. 1) by using a $4{ imes}4$ Latin square design. Among the essential amino acids, apparent ileal digestibility (AID) of Met and Val were increased in pigs fed FSP diets compared with those fed non-FSP diets (p<0.05). AID of Met, Phe and total essential amino acids were higher in pigs fed complex diets than in those fed simple diets (p<0.05). Among the non-essential amino acids, AID of Ala in FSP treatments was greater than that in non-FSP treatments (p<0.05). In addition, Asp, Cys, Glu, Pro, Ser and total non-essential amino acid digestibilities in pigs fed complex diets were higher compared with those fed simple diets (p<0.05). Interaction was observed in AID of Met, Asp and Pro. In conclusion, these results indicated that feeding of 5% FSP to nursery pigs improved feed efficiency and AID of amino acids, and diet complexity did not maximize the growth performance of pigs in the subsequent phase.