Background: There are numerous variations in root and root canal morphology of mandibular first permanent molar. This study was carried out to assess the incidence of three roots and four canals in clinical cases of root-treated mandibular first molars of Saudi Arabian sub-population in Jeddah. Materials and Methods: In this clinical study, a total of 181 treated permanent mandibular first molars were studied. All the teeth examined clinically under magnification and radiographically. Results: The results showed that the fourth canal was found in 58.01%. 1.65% had five root canals in which the fifth root canal present as a middle mesial canal. Also, 7.18% of the examined teeth had additional distolingual root, which is called radix entomolaris. Conclusions: The frequency of four root canals in the mandibular first molar of Saudi sub-population in Jeddah was high whereas the occurrence of the fifth canal was relatively rare. Careful inspection of fourth canal in mandibular first molars is essential for successful root canal therapy.