It may be beneficial, although risky, to decide to trust others during social interactions in daily life. The current study explored the time course of the effect of trust decisions on outcome evaluation processing using electroencephalographic (EEG) techniques. Twenty-two healthy young adults made trust or distrust decisions regarding a message they received from a stranger in a card game. The results showed that at the early stage of outcome processing, feedback-related negativity (FRN) was more negative for losses than for gains, and more importantly, this valence effect of FRN was greater for outcomes following trust decisions than distrust decisions. At the later stage, the P3 was larger for gains than for losses and for outcomes following trust decisions than distrust decisions. In addition, at both the early and late stages, differences in the mid-frontal theta power between losses and gains were also greater in the trust condition than in the distrust condition. These findings indicate that trust decisions could effectively modulate the processing of outcome evaluation at different stages. The trust effect at the early stage of outcome processing suggests that compared to distrust decisions, trust decisions result in greater motivation and expectancy to obtain positive outcomes. While the trust effect on the later stage of outcome processing suggests that individuals pay more attention and feel stronger emotional experience to the outcomes following trust decisions. This study sheds light on the neural mechanisms of trust decisions modulating outcome processing.