Mirror neuron system can be activated by observation and execution of an action.It has an important function of action understanding.We investigated brain activations in humans by observing the strength of a hand grasp using functional magnetic resonance imaging.Twenty right-handed healthy individuals,consisting of 10 males and 10 females,aged 22.40 + 2.04 years,were recruited into this study from September to November 2017 via posters.Light hand grasp task video showed a hand lightly grasping and releasing a ball repeatedly.Powerful hand grasp task video showed a hand tightly grasping and releasing a ball repeatedly.Functional magnetic resonance imaging block design paradigm comprised five stimulation blocks altating with five baseline blocks.Stimulation blocks were presented with two stimulus tasks,consisting of a light grasp and a powerful grasp.Region of interest was defined around the inferior parietal lobule,inferior frontal gyrus,and superior temporal sulcus which have been called mirror neuron system.The inferior parietal lobule,fusiform,postcentral,occipital,temporal,and frontal gyri were activated during light and powerful grasp tasks.The BOLD signal response of a powerful grasp was stronger than that of a light grasp.These results suggest that brain activation of the inferior parietal lobule,which is the core brain region of the mirror neuron system,was stronger in the powerful grasp task than in the light grasp task.We believe that our results might be helpful for instructing rehabilitation of brain injury.This study was approved by the Institutional Review Board of Daegu Oriental Hospital of Daegu Haany University on September 8,2017 (approval No.DHUMC-D-17020-PRO-01).