[Purpose] This study examined whether motor imagery leads to a decrease in the temporal process in terms of the onset of muscle activation and reaction time according to acquisition of motor skills in a serial reaction time (SRT) task. [Subjects] Forty one healthy, right-handed subjects with no history of neurological, orthopedic, or psychiatric disorders were enrolled in this study. The subjects were assigned randomly to the motor training group (n=13), motor imagery group (n=14), and control group (n=11). [Methods] After six visual stimuli, the subjects were instructed to move or press a moveable arm/button according to the corresponding stimuli. However, the motor imagery group performed the task without actual movement in the same task paradigm. The kinetic parameters (i.e. muscle activation and movement initiation) were analyzed before and after the training/controlled session over three consecutive days with two repetitions per day for each group. [Results] After motor skill acquisition, the motor training group and motor imagery group showed a significant decrease in processing times between the visual stimuli and two predetermined onsets, which consisted of the onset of muscular activation and reaction time. However, there were no significant changes in the control group. [Conclusion] The decrease in processing time through motor imagery can be attributed to the rapid onset of muscle activation and movement initiation, which might be induced by neuromuscular adaptation in the motor performance phase. Furthermore, we assumed that imagining the performance of a motor task could contributes to improving the motor performance in motor sequential learning.