This study compared the effects of the reading-while-listening condition and the reading-only condition on English as a foreign language (EFL) learners' acquisition of the four dimensions of vocabulary knowledge: form recognition, grammar recognition, meaning recall, and collocation recognition. Accordingly, 24 words within four frequency groups were selected and total 60 Chinese EFL students were enrolled. The results revealed that new words could be learned incidentally in both conditions. However, more words were learned in the reading-while-listening condition. A significant effect of word exposure frequency on incidental vocabulary gains was observed for acquisition of word form and grammar, but no significant effect was found for acquisition of meaning and collocation. Although the reading-while-listening condition was more effective for incidental vocabulary acquisition than the reading-only condition, it appeared that for these two conditions to be a valuable source for incidental vocabulary acquisition, both word exposure frequency and elaborate word processing were needed.