Abstract: Objective: To investigate the effect of erythromycin on the induced rat periapical periodontitis.Methods: The experiment animal model of periapical inflammation was established with SD rats. Saline and erythromycin were placed in canal respectively after root instrumentation and rats were killed at 1,3,and 7 days after operation. Microscopical analyse of the periapical tissue were performed according to histological criteria. Results: At the 3d and 7d period, the degree of the inflammation reaction and inflammatory cell infiltration in erythromycin group was significants lighter than that in Saline group.Conclusion: Erythromycin could control the induced rat periapical periodontitis effectively by inhibiting the inflammation reaction and inflammatory cell infiltrationin the active phase.