This study aimed to examine how imaginative capabilities act as a mediator between personality traits and academic performance, and to compare the different ways these factors affect engineering and science majors. The hypotheses were tested with data from five universities across different regions in Taiwan. The participants were a sample of 700 undergraduates, who were divided into engineering and science groups. The results showed that, through the mediator effect of imaginative capabilities, conscientiousness had the greatest impact on academic performance among engineering and science groups. Agreeableness was identified as the second major predictor of student performance in both groups. The influence of openness on student performance was slight and positive, whereas that of extraversion was slight and negative. The major differences in personality, imagination and performance structures between the two groups were the direct results of agreeableness, extraversion, initiating imagination, and conceiving imagination.