Objective To compare the initial penetration depth of nickel-titanium（NiTi） and stainless-steel（SS）spreader during lateral compaction and the quality of the seal in curved canals. Methods Forty extracted mandibular premolars with a single curved canal were divided into two groups: no more than 20° and more than 20° based on degree of curvature. All canals were instrumented using a rotary instrumentation technique. NiTi and SS spreaders were used to obturate the canals containing a master cone while the penetration depths were measured. Horizontal sections were cut in 2 and 4 mm from the apex and photographed under stereomicroscope. The percentage of gutta-percha-filled are（PGP） of cross-sections was measured using an image analysis program. Results In canals of more than 20°, the penetration depths and PGP of 2 mm from the apex of NiTi spreaders were higher than SS spreader. In canals of no more than 20°, there were no significant difference between them（P>0.05）. At 4 mm from the apex, there was no significant difference between two groups. Conclusion NiTi spreaders has a higher penetrated depth and obturation density than SS spreaders in severed curved canals.
