In the present study, we examined the relation of knowledge of semantic radicals to students' language proficiency and word reading for adult Chinese-as-a-foreign language students. Ninety-seven college students rated their proficiency in speaking, listening, reading, and writing in Chinese, and were administered measures of receptive and productive knowledge of semantic radical position and function. A latent variable for language proficiency was constructed based on the number of semesters in Chinese courses, and students' self-rating in Chinese. Two language proficiency groups were reliably formed. Students with higher language proficiency had significantly better performances on all the measures of knowledge of semantic radicals and word recognition. Furthermore, the receptive and productive knowledge of semantic radical position and productive knowledge of semantic radical function were positively related to word reading after controlling for proficiency level. Productive knowledge of semantic radical function was uniquely and positively related to word reading after accounting for proficiency level and other measures of semantic radical knowledge.