Objectives: The aim of this study was to assess leakage of root canals obturated using different techniques: cold lateral condensation, single-cone technique and Thermafil technique under controlled vacuum conditions. Material and Methods: Forty-nine single-rooted teeth were divided into three groups of 15 samples each, and positive and negative control group with two samples in each of them. Two groups were instrumented using manual (Step-back) technique and obturated by cold lateral condensation and Thermafil techniques. Third group was instrumented using rotary (ProTaper) technique and obturated with single-cone guttapercha. After a one-week setting period, the samples were covered by insulating varnish and wax and placed in a vacuum flask containing methylene blue dye. The air was evacuated with a vacuum pump to a pressure of 75 torr. The samples were sectionated longitudinally and the linear extent of dye penetration was measured. Results: Group mean linear dye penetration values were: cold lateral condensation 4.67 ± 4.19 mm, single-cone 3.61 ± 4.36 mm, and Thermafil 1.17 ± 0.86 mm. The results indicate significantly lower dye-penetration in samples obturated using Thermafil technique, when compared to the samples obturated using cold lateral condensation. Conclusion: Even though Thermafil technique resulted in the least amount of a dye- penetration, all obturation techniques showed apical leakage.