Objective·To observe the three dimensional morphology of root canal of mandibular first premolars with single canal for individuals at different ages. Methods·Ten mandibular first premolars with single canal that needed to be extracted due to orthodontic reason from individuals aged 15-25 years (the young group) and ten mandibular first premolars with single canal that needed to be extracted due to periodontal diseases from individuals aged 55-75 years (the elderly group) were selected. Data were gathered by Micro-CT scanning and CTAn software was used to analyze root volume, canal volume, canal area, and structure model index (SMI). Results·Average canal volume and area were (12.308±4.550) mm3 and (48.869±8.064) mm2 in the young group and (3.798±0.944) mm3 and (25.187±5.072) mm2 in the elderly group. The differences were both statistically significant (P<0.05). The differences in average volume fractions of middle and apical third of root canals were statistically significant (P<0.05). The elderly group has a significantly higher SMI than the young group (P<0.05). Conclusion·For the treatment of patients at different ages, we need to thoroughly understand anatomic characteristics of root canals, pay more attention to root canal preparation, so as to improve the success rate of root canal treatment.