This study examined the predictive role of several learner factors in second language (L2) incidental vocabulary acquisition through reading: L2 proficiency, motivation, anxiety, and mastery of strategies. Participants were 129 English learners in a comprehensive university in China. Participants read two English texts and were given an unannounced vocabulary test to assess their incidental vocabulary acquisition. Their levels of motivation, anxiety, and mastery of strategies were measured by three specifically designed instruments that targeted incidental vocabulary acquisition. A multiple linear regression analysis revealed that L2 proficiency, anxiety, and mastery of strategies were positive and significant predictors of incidental vocabulary acquisition, although motivation was not. This confirms the role that learners' linguistic, affective, and cognitive features play in L2 incidental vocabulary acquisition.