In this study, we investigated the contribution of morphological awareness (MA) in one language to word reading and vocabulary in another language in Hong Kong Cantonese-English speaking children, who learn Chinese and English in school in parallel from the age of 3.5 years onwards. Our sample consisted of 97 Cantonese-English speaking children including 34 first graders, 28 second graders, 21 third graders and 14 fourth graders. All children were administered tasks of nonverbal reasoning, phonological awareness in Chinese, and lexical compounding, vocabulary, and word reading in both Chinese and English. Results revealed that second language (L2) English MA significantly contributed to first language (L1) Chinese word reading and Chinese vocabulary knowledge. However, L1 Chinese MA was not uniquely associated with either L2 English word reading or vocabulary knowledge. Findings suggest that among Chinese children learning in Chinese medium of instruction schools with English taught as a second language, compounding skills in English may be useful for facilitating Chinese word reading and vocabulary acquisition, but Chinese compounding skills are not uniquely important for learning in English.