The ability to recognize oneself in voluntary action is called the sense of agency and refers to the experience of causing one's own actions and their sensory consequences. This form of self-awareness is important not only for motor control but also for social interactions and the ascription of causal responsibility. Here, we examined the sense of agency at early and prereflective stages of action perception using ERPs. Subjects performed a visual forced-choice response task in which action effects were either caused by the subject or by the computer. In addition, to modulate the conscious experience of agency, action effects were subliminally primed by the presentation of congruent, incongruent, or neutral effect stimuli before the action. First, we observed sensorimotor attenuation in the visual ERP selectively for self-generated action effects. That is, the N1 component, a negative deflection around 100 msec after a visual stimulus, was smaller in amplitude for visual effects caused by the subject as compared with effects caused by the computer. Second, congruent effect priming enhanced the explicit judgment of agency and further reduced the N1 amplitude for self-generated effects, although effect primes were not consciously processed. Taken together, these results provide evidence of a top-down modulation of sensory processing of action effects by prior effect information and support the neurophysiological mechanism of sensorimotor attenuation as underlying self-registration in action. Our findings suggest that both efferent information and prior thoughts about the action consequence provide important cues for a prereflective form of the experience of being an agent.