Abstract: Effect of lysine content in a low-crude-protein fodder on the growth performance and serum biochemical indices of 70-to-100-kg-class gilts was studied. Ninety-six gilts crossbred between Landrace and Large White with an average body weight of (69.01 ±4.43)kg and similar physical conditions were randomly divided into 4 groups with 4 replicates each for the study. With an equal energy and balanced amino acid design, the digestible lysine (DLys)at 0.61% (Group Ⅰ), 0.66% (Group Ⅱ), 0.71% (Group Ⅲ) or 0.76% (Group Ⅳ )in the low protein diets were applied for the feeding. The results showed the gilts in Group Ⅱ had the highest average daily gain at 908.33 g·d^-1 and the lowest feed/gain ratio at 2.77 (P < 0.05) among all. The Group Ⅱ gilts also showed a significantly lower serum urea nitrogen than the gilts in Group Ⅰ or Group Ⅳ (P < 0.05), but no significant difference in comparison with Group Ⅲ (P>0.05). The serum insulin and insulin growth factor-Ⅰ in all test animals increased initially, and then, decreased as DLys increased (P>0.05). The serum lysine and threonine of the Group Ⅰ gilts were significantly lower than those of Group Ⅳ (P < 0.05), while the other amino acids decreased initially and followed by an increase with increasing DLys(P>0.05), which was lower in Group Ⅱ. Taking all factors into account, a daily DLys intake of 16.57 g·d^-1(or, as in Group Ⅱ, 0.66% DLys in the low-protein diet) would be optimal for the growth of gilts in the weight class.