the purpose of this study was to compare the apical seal mof thermafil with lateral and vertical condensation techniques in mesiobuccal root of maxillary first molars.
245 upper first molarteeth were extracted.among them 40 teeth  were selected having 20-25 degrres of curve(according to schnider analysis) in apical one their of their mesiobuccal roots.these teeth were also selected so that the first measuring fill in all species was a number 15 file.then the selected teeth were cut from CEJ by a metallic disc and the mesiobuccal roots were prepared up to number 30 file using step back technique.after this stage the teeth were divided into 5 gtoups.group A:10 teeth that their mesiobuccal canals were obturated by lateral condesation techmique.group B:10 teeth that their mesiobuccal canals were obturated by vertical condesation techmique.group C:10 teeth that their mesiobuccal canals were obturated by thermafil.groupD:5 teeth as negative control group.group E:5 teeth as positive control group.after coating the negative control roots by 2 layers of nail polish completely and also the teeth of group A,B,C,D nad E by 2 layers of nail polish except of their apical 1.5 mm,the teeth were incubated and placed within india ink vertically for 48 hours.then the mesiobaccal roots were sectioned buccolingually and the amount of dye penetration was measured by stereomicroscope.the data were evaluated by statistical analysis(kruskal wallis).the apical seal from thermafil was significantly lower than lateral and vertical condesation techniques in mesiobuccal roots.