This study investigated the effects of semantic transparency of Chinese suon Chinese as a second language (CSL) learners' incidental learning of word meanings in sentence-level reading and passage-level reading. The accuracy of the learners' lexical inferencing was compared among various types of words (transparent, semi-transparent, and opaque words), different context lengths (sentence and passage contexts), and learners with different L1 backgrounds (with and without a Chinese character background in their L1s). In the study, ninety CSL adult learners were asked to infer the meanings of target words in the sentence context and the passage context. The results indicated that the effects of semantic transparency and context length on inferencing accuracy were significant, while the effect of L1 background was not. It was also found that there were significant interactions between transparency and context length as well as between transparency and L1 background.