Approaches to learning a second/foreign language can be affected by such individual differences as personality traits, motivational variables, and task difficulty. Personality traits have been proven to influence academic achievement, but their effects on motivation and language learning approaches remain unanswered. The first purpose of the study aims to investigate whether L2 grit predicts university students' learning approaches and how far expectancy of success and task difficulty perception mediate the relationship between L2 grit and the learning approaches. This study also examines the extent to which English proficiency has an indirect effect on the learning approaches. Five hundred eighty-six EFL university students taking English courses in Taiwan participated in the study. The structural model showed that perseverance of effort directly, strongly, and positively predicted deep strategies, and the mediation analysis revealed a partially mediated effect of expectancy of success. By contrast, consistency of interest directly and negatively predicted surface strategies, and the mediation analysis revealed a partially mediated effect of task difficulty perception. Students' English proficiency has an indirect effect on surface strategies through the mediation of expectancy of success.