The P300 in event-related potentials (ERPs) has been implicated in outcome evaluation and reward processing, but it is controversial as to what aspects of reward processing it is sensitive. This study manipulated orthogonally reward valence, reward magnitude, and expectancy towards reward magnitude in a monetary gambling task and observed both the valence and the magnitude effects on the P300, but only when the amount of reward was expected on the basis of a previous cue. The FRN (feedback-related negativity), defined as the mean amplitudes of ERP responses to the loss or the gain outcome in the 250-350 ms time window post-onset of feedback, was found to be sensitive not only to reward valence, but also to expectancy towards reward magnitude and reward magnitude, with the violation of expectancy and the small magnitude eliciting more negative-going FRN. These findings demonstrate that while the FRN may function as a general mechanism that evaluates whether the outcome is consistent or inconsistent with expectation, the P300 is sensitive to a later, top-down controlled process of outcome evaluation, into which factors related to the allocation of attentional resources, including reward valence, reward magnitude, and magnitude expectancy, come to play.