Objective:To evaluate the incidence pain after treatment of acute periapical periodontitis using calcium hydroxide and CP as an intracanal dressing at the first time.Methods:303 patients with single-root acute periapical periodontitis were randoml allocated to three groups.A group was treated by stainless steel hand files and opened;B group was treated by stainless steel hand files and covered by CP.;C group was treated with Hero nickel-titanium rotary instruments and covered by calcium hydroxide paste.The incidence and degree of incidence pain after root canal preparation in the three groups were recorded.Results:There were significant difference between group A and group B

group B and group C

group A and group C(P<0.05).Conclusion:Nickel-titanium rotary instruments and calcium hydroxide paste used can effectively prevent and reduce the pain and dropsy during root canal therapy on acute periapical periodontitis.