Objective To investigate the prevalence and treatment of radix entomolaris（RE） in mandibular first molars. Methods 378 cases of mandibular first molars with root canal therapy were collected. The case included clinical records and periapical radiographs of preoperative, intraoperative and postoperative. The root number, root canal number and root canal therapy were investigated. Results The RE prevalence of mandibular first molars was 26.98％ （102/378）. Both gender and bilateral occurrence of RE did not show a significantly difference（P>0.05）, but there was a significantly greater incidence of instrument separation, lateral perforation and underfill of the root canals in RE than other canals in mandibular first molars（P
