A 60-d feeding trial was conducted to determine the dietary leucine requirement of juvenile sea cucumber Apostichopus japonicus with initial body weight (16.40±0.14) g. Six experimental diets were formulated with the graded leucine levels 1.29% (D1, control group), 1.63% (D2), 1.98% (D3), 2.22% (D4), 2.58% (D5) and 2.97% (D6) dry diets. The results showed that: There were no differences in survival rate among all groups (96.00%-98.67%). Both weight growth rate (WGR) and the specific growth rate (SGR) increased with increasing leucine content until reaching peak levels at 1.98% dietary leucine, but decreased thereafter. The WGR and SGR of D3, D4 and D5 groups were significantly higher than D1 group. The WGR reached maximum value of 100.84% in the D3 group. There were no significant effects on ratio of intestine weight to body wall weight (IBR) and ratio of intestine length to body length (IBL) of sea cucumber. The crude lipid contents of body wall was increased when dietary leucine content increased from 1.29% to 1.98% but decreased when the dietary leucine content exceeded 1.98%, and D3 group was significantly higher than other groups, but there were no significant effects on moisture, crude protein and crude ash contents. Both methionine and leucine content of body wall were significantly increased by dietary leucine, but there were no significant effects on total amino acids (TAA). Both lipase and protease activities of intestinal were increased with increasing levels of leucine up to 1.98% diet and then decreased. The lipase of D3 group was significantly higher than other groups, meanwhile, the protease of D2, D3 and D4 groups were significantly higher than other groups. All of alanine aminotransferase (ALT), asparate aminotransferase (AST) and total antioxidant capacity (T-AOC) activities of intestinal were increased when dietary leucine content increased from 1.29% to 1.98%. There were no significant effects on AST activity when the dietary leucine content exceeded 1.98%, but the ALT and T-AOC activities were decreased. The T-AOC activity of D3 group was significantly higher than D1, D2, D5 and D6 groups. The catalase (CAT) and superoxide dismutase (SOD) activities reached maximum value when the dietary leucine content was 2.22%. The CAT activity of D4 was significantly higher than D3, D5 and D6 groups, The SOD activity of D4 was significantly higher than D1, D2 and D6 groups. The malondialdehyde (MDA) content was decreased when dietary leucine content increased from 1.29% to 1.98% but increased when the dietary leucine content exceeded 2.22%, the MDA contents of D3 and D4 groups were significantly lower than other groups. With WGR as evaluation indicator, quadratic regression analysis showed that the optimum dietary leucine requirement of sea cucumber with body weight 16.40 g was 2.11% diet (10.37% dietary protein).