This study focuses on native speakers and non-native speakers' knowledge about the occurrence and syntactic constraints of topic-structures in Mandarin and Cantonese. The preliminary findings show that while native speakers are sensitive to the clause structures and the syntactic constraints of topic-structures in the matrix and embedded clauses, non-native speakers' understanding of where topic-structures may occur in a sentence is limited, resulting in their poorer performance in the survey. In addition, based on the survey date, the study argues against the view that topic-structures are loosely placed at the beginning of a sentence.