OBJECTIVE:Straight or slightly curved single root canals of teeth (<20 degree) were instrumentated with three commonly the hand endodontic instruments,which included K reamer,K file and H filel.The shape of prepared canal and root canal deviation at three levels were measured by computerized digital image processing program.The results demostrated that any significant differences were not found on root canal deviation in either labio-lingual or mesiodistal direction at each level,and that the shape of prepared canal was not different significantly.Apical transportation of the severely curved artificial root canals(25-30 degree) showed that H file was the most severe,K reamer was the least.It is suggested in this article how we select endodontic instruments rationally and effectively.