This study was conducted to investigate the effects of L-arginine (L-Arg) supplementation on performance, serum amino acid concentrations and immune biochemical indexes of lactating sows. A total of 32 healthy Yorkshire譒andrace sows (2 parities) with similar body weight were randomly assigned into 4 groups with 8 replicates per group and 1 sow per replicate. The sows were fed a basal diet added with 0 (control group), 0.8%, 1.0% and 1.2% L-Arg, respectively. The experiment lasted for 21 days, which was from delivery day to day 21 of lactation (weanling).The results showed as follows: on day 10 and day 18 of the experiment, except that there was no significant difference in serum ornithine concentration between 1.0% L-Arg group and the control group (P>0.05), L-Arg supplementation significantly increased serum arginine, ornithine and proline concentrations of lactating sows (P<0.05 or P<0.01), and there were the best results in 1.0% and 1.2% L-Arg groups among all groups. Total protein concentration in serum of lactating sows was not significantly affected by L-Arg supplementation on day 10 and day 18 of the experiment (P>0.05). However, on day 10 and day 18 of the experiment, the concentrations of albumin, IgM and IgG in serum of lactating sows in L-Arg groups were significantly increased (P<0.05) and serum urea nitrogen concentration of lactating sows was significantly decreased compared with the control group (P<0.05), and serum urea nitrogen concentration in 1.0% L-Arg group was the lowest among all groups, but there was no significant difference in serum urea nitrogen concentration among L-Arg groups (P>0.05). Weaning litter weight, piglet weight at weanling and average daily gain were significantly increased with the supplementation of L-Arg to lactating sows (P<0.05), and those in 1.0% L-Arg group were the highest among all groups, but there were no significant differences among L-Arg groups (P>0.05). The interval days from weanling to estrus of sows in 1.0% L-Arg group were significantly shorter than those in the other three groups (P<0.05), and there were no significant differences among the control group, 0.8% L-Arg group and 1.2% L-Arg group (P>0.05), but those had a increasing tendency as dietary supplemental level of L-Arg increased. Regression analysis showed that serum amino acid concentrations of sows were increased linearly with the increase of dietary L-Arg supplemental level (P<0.05), but performance and serum immune biochemical indexes of sows were not significant regression relation with dietary L-Arg supplemental level (P>0.05). In conclusion, L-Arg supplementation can improve partial amino acid concentrations, metabolism and immune function of lactating sows so as to get a better growth performance of suckling piglets and get shorter interval days from weanling to estrus of sows. In this experiment, the optimal supplemental level in the diets for lactating sows is 1.0%.