Objective: To compare the initial penetration depth of nickel-titanium (NiTi) and stainless steel (SS) spreader during lateral compaction of .04 and .02 tapered master gutta-percha cones and the quality of the canal seal. Methods: Forty extracted mandibular premolars with a single curved canal were prepared using standardised rotary instrumentation technique. NiTi and SS spreaders were used to obturate the canals containing a .04 or .02 master cone while the penetration depths were measured. Horizontal sections were cut 2 and 4 mm from the apex and photographed under a stereomicroscope. The area of the canal and gutta-percha in cross-section was measured using an image analysis program. Results: NiTi spreaders penetrated to a significantly greater depth than SS spreaders (p 0.01). At the 2-mm level, there was significantly more gutta-percha using the .02 tapered gutta-percha cone in the NiTi spreader group than in the SS spreader group (p 0.05), but no significant difference occurred using .04 tapered gutta-percha (p > 0.05). At the 4-mm level, there was no significant difference in obturation density either between the NiTi spreader and the SS spreader or between the .02 and .04 tapered gutta-percha (p > 0.05). Conclusion: NiTi spreaders penetrated to a significantly greater depth than SS spreaders and the area occupied by gutta-percha was significant greater when filling curved canals with .02 tapered gutta-percha. Keywords: lateral condensation, master cone, nickel-titanium, spreader