According to the literature, “C-shaped” canals occur mainly in the mandibular second molar. As indicated in the article by Weine (1998), the incidence of “C-shaped” canals in the mandibular second molar was 4%~7.6%. Due to complications of anatomical deviation, there can be very much dead space in “C-shaped” canals, which are not easily debrided by instruments, and may frequently cause a poor prognosis for treatment. But there is much less literature discussing “C-shaped” canals in mandibular second premolars. Owing to limitations of the operating view and working field a “C-shaped” canals in a mandibular second premolar is more difficult to manage. This article is a case report about a “C-shaped” canals in a mandibular second premolar.