Objective       To determine the effect of the size of preparation and the temperature of obturation on apical leakage of root canal in vitro.       Methods       A number of healthy premolars with single canal were collected. The root canals were prepared with Protaper nickel titanium instruments and divided into group F1 to F6 (F6 meant 0.60 mm

tapered 0.06

to which the root canals were prepared with K files by step back technique after prepared to F5) according to the finishing files. Teeth of every group were divided into low and high temperature subgroups according to the obturation temperature

with 7 teeth in each subgroup. The surfaces of the teeth were coated with 2 layers of nail polish except the apical 3 mm of the roots. All teeth were stained and made to transparent teeth

and the depth of dye from apices was measured and recorded under a dental microscope. The results were analyzed statistically.       Results       The effect of obturation temperature on apical microleakage had no significant difference (P0.05)

but the effects of root canal preparation sizes (F1 to F6) on apical microleakage had significant differences (P0.05)

and there was no interaction between 2 factors of obturation temperature and preparation sizes (P0.05). There was significant difference in apical microleakage between F2 and any other group (P0.05)

no significant difference between F1 and F3 or F4 (P0.05)

but there was significant difference between F1 and any other groups (P0.05); there were significant differences of F6 with F1 to F3 groups (P0.05) and no significant difference of F6 with F4 or F5 group (P0.05).       Conclusion       Obturation temperature has no effect on apical microleakage

and root canal preparation size has effect on apical microleakage. Apical microleakage will be minimum when the preparation size is F2.