The mirror neuron system (MNS) is activated when observing the actions of others. However, it remains unclear whether the MNS responds more strongly to natural bodily actions in the observer's motor repertoire than to unnatural actions. We investigated whether MNS activity is modulated by the unnaturalness of an observed action by inserting short pauses in the middle of the action (0, 2, and 6 pauses; no-pause, pause-1, and pause-2 conditions, respectively). The results indicated that the number of pauses significantly affected motor area activity. Subsequent analyses revealed significant differences between the pause-1 and pause-2 conditions (P less than 0.01), as well as the no-pause and pause-2 conditions (P less than 0.05). There was significant activation in the pause-1 condition (P less than 0.001), while significant deactivation was observed in the pause-2 condition (P less than 0.05). These results indicate that MNS activity is modulated by the kinematic characteristics of the observed action. We suggest the possibility that a slightly deviated action may enhance the MNS activity during action observation, while a highly unnatural action would lead to a strong attenuation (deactivation) of activity in the MNS. (Contains 6 figures and 1 table.)