Endodontic treatment is directed toward the prevention or curing apical periodontitis. Since the main etiology of apical periodontitis is bacterial infection, the primary goals of root canal therapy are to eliminate microorganisms from the root canal system and to prevent reinfection via micro leakage. Bacterial elimination is accomplished through chemo-mechanical preparation. Although microbiological studies have shown that larger apical preparation sizes may lead to a greater reduction in remaining bacteria, but there are two concerns in this regard: the impact of final canal shape and size on root strength and the impact of apical enlargement on the outcome of endodontic treatment. There is a reverse relationship between the size of root canal preparation and the two aforementioned factors. In the present study, we evaluated the effect of canal size, both coronally and apically on the amounts of micro-leakage using a new quantitative method: glucose penetration. Since leakage occurs through two surface areas, increasing the surface area may lead to an increase in micro-leakage.