Lactobacillus plantarum (LP) contributes to the intestinal health, whereas the study about the effects of LP on Pekin ducks is lacking. This study aimed to investigate the effects of LP on growth performance and intestinal health of Pekin ducks. A total of 180 one-day-old birds were randomly allotted to 3 treatments, and ducks were fed with a basal diet (Control) or basal diet supplemented with 400 (LP1) and 800 (LP2) mg/kg LP (5 × 109 CFU/g). The animal trial lasted for 42 d. Results showed that the LP1 and LP2 treatments improved growth performance (feed conversion) of ducks during the period of 1 to 42 d. At the end of 21 d, the decreased serum levels of interleukin (IL)-1β, interferon (IFN)-γ as well as downregulated ileal mRNA expression of IL-1β were observed in 2 doses of LP group. Meanwhile, the ileal mRNA levels of major histocompatibility complex (MHC)-II, IL-4, Claudin, Occludin were upregulated with 2 doses of LP supplemented. In addition, both LP treatments increased the relative abundance of Firmicutes and decreased Bacteroidetes wherein the relative abundance of Bacteroides fragilis was dropped parallelly. It is worth mentioning that markedly increased secretory immunoglobulin A content in ileal mucosa was observed in the LP1 group at d 21. At the end of the trial, the levels of serum complement 3 and β-defense were elevated with 2 doses of LP treated. Additionally, the ileal mRNA expressions of MHC-II, lysozyme were upregulated, and the diversity of the flora was also improved in the LP1 and LP2 groups. In conclusion, dietary LP improved the growth performance and intestinal health of Pekin ducks, and 400 mg/kg LP seemed to work better.