A total of 144 Duroc × Landrace ×Yorkshire crossbred weaned piglets at the age of 21 days were selected and randomly divided into 4 treatment groups, with 6 replicates per group and 6 piglets per replicate. Piglets in the control(CTL) group and lipopolysaccharide(LPS) group were fed a basal diet, the low-dose rosemary extract group(RE200) and the high-dose rosemary extract group(RE400) were fed a basal diet supplemented with 200 mg/kg and 400 mg/kg rosemary extract(RE), respectively, for 28 days. 100 μg/kg LPS was intraperitoneally injected to piglets in the LPS, RE200, and RE400 groups on day 21 and day 28, respectively, and those in CTL group were injected with the same dose of normal saline. The results were shown as follows. 1) There were no significant differences in growth performance indexes among all groups(P>0.05). 2) In LPS group, the superoxide dismutase(SOD) activity was markedly reduced (P<0.05), the aspartate aminotransferase(AST) activity and interleukin-6(IL-6) mass concentration were markedly elevated(P<0.05) compared to CTL group. In RE200 group, the malondialdehyde(MDA) concentration and AST activity were markedly reduced(P<0.05), and the index of total antioxidant capacity(T-AOC) concentration was markedly elevated(P<0.05) compared to LPS group. In RE400 group, the AST activity and IL-6 mass concentration were markedly reduced(P<0.05) compared to LPS group. 3) In LPS group, both the villus height and villus-crypt ratio of ileum were markedly reduced(P<0.01), while the crypt depth was markedly elevated(P<0.01) and diamine oxidase(DAO) activity was markedly elevated(P<0.05) compared to CTL group. In RE200 and RE400 groups, both the villus height and villus-crypt ratio of ileum were markedly elevated (P<0.01), while the crypt depth was markedly reduced(P<0.01) compared to LPS group, and DAO activity in RE200 group was markedly reduced(P<0.05) compared to LPS group. 4) The α-diversity analysis indicated no significant variations across all groups(P>0.05), and the results of β diversity analysis showed that the bacterial community structure was basically similar across all groups. In LPS group, the relative abundance of Romboutsia was markedly elevated(P<0.05) compared to CTL group. In RE200 group, the relative abundance of Romboutsia was markedly reduced(P<0.01), and the relative abundance of Lachnospiraceae UCG-004 was markedly elevated(P<0.01) compared to LPS group. In conclusion, the addition of RE to the diet can significantly improve antioxidant capacity, reduce gut permeability, alleviate gut damage, and improve gut flora composition of weaned piglets. Under the condition of this experiment, the suitable addition level of RE in the diet of weaned piglets is 200 mg/kg.