The objective of this study was to evaluate the capability of sonic vibration and continuous irrigation with various irrigant solutions to remove the smear layer formed by nickel-titanium (Ni-Ti) rotary instruments. Root canals were prepared in 56 single-rooted human mandibular premolars with no dental caries by using NiTi rotary instruments. The teeth were divided into six experimental groups: control (Co) group, irrigated with sterile physiological saline only (8 teeth); SH group, irrigated with 5.25% sodium hypochlorite (NaOCl) solution (10 teeth); SC group, irrigated with 3% ethylene diamine tetraacetic acid (EDTA) solution (10 teeth); Co+Vi group, irrigated with sterile physiological saline along with sonic vibration (8 teeth); SH+Vi group, irrigated with 5.25% NaOCl solution along with sonic vibration (10 teeth); and SC+Vi group, irrigated with 3% EDTA solution along with sonic vibration (10 teeth). The surface walls in the central portion of the coronal and apical halves of the root canals in experimental groups were examined with a scanning electron microscope (SEM), and the mean numbers of open dentinal tubules after root canal irrigation were calculated. There was no statistically significant difference between the SH and SH+Vi groups, but cleaning capability improved significantly in the coronal area in the SC+Vi group compared with the coronal side in the SC group. In this study, we confirmed that continuous irrigation of the root canals using EDTA along with sonic vibration is effective in removing the smear layer formed by Ni-Ti rotary instruments.