In various studies it has been shown that major part of effective factors in root canal therapy failure is error during treatment. for example, in washington study nearly 76% of treatment failure factors are related to error during treatment. Considering that most of the errors are preventable, this study took palce for evaluating the errors during root canal therapy by students of Isfahan dental school and presenting suitable preventive strategies in order to increase the treatment success.In this study 1260 radiography of root canal treated teeth by students of semister 11 and 12 in years of 1999-7 were studied, and errors of each were diagnosed and registered in a checklist, at least with statistical analysis of collected informations, these results were obtained: In 1260 ispected cases,2752 errors were seen, that in average every tooth has 2.2 errors. Vertical gap with 430 cases was maximum and preparated but unfilled canal and middle root perforation with one case were seen rarely. Average error in each case in upper arch was 2.14 and in lower arch was 2.22 (P=0.35).Cosidering that incomplete obturation (include vertical gap and inadequate obturation) is the major treatment error in this study and taking in view that incomplete obturation in many studies such as Washington study is the major couse of treatment failure, so with regarding the correct treatment principle, should be prevented as far as possible.