Self-efficacy toward science learning has been shown to play a crucial role in determining students' motivation and achievements. Social cognitive theory proposes that positive and negative task outcomes affect mastery experiences from which self-efficacy develops. The current research examined whether prior level of self-efficacy would serve as a moderator of the effect of experiential valence on self-efficacy in science learning. One hundred and thirty engineering undergraduates with varying levels of prior self-efficacy (high, medium, and low) were randomly assigned to receive either a positive or a negative task experience regarding circuit design. The findings of our experiment showed that students with lower levels of self-efficacy appeared to be more affected by positive versus negative task experiences, and those with higher levels of self-efficacy tended to be more affected by negative versus positive task experiences. The present findings indicate that both valence of task experience and students' prior self-efficacy affect their changes in self-efficacy with regard to STEM learning. The present findings have far-reaching implications for enhancing self-efficacy on learning of science.