The exclusive expression weiyi 'sole/only' in Mandarin can appear preverbally in a relative clause (e.g., wo weiyi xihuan de shu 'the sole book I like') but not in other types of clauses (e.g., *Wo weiyi xihuan shu, intended: 'I only like books'). This paper first justifies the claim that weiyi may not only function as an adjectival modifier but also appear preverbally inside a relative clause, and then demonstrates how weiyi is related to definiteness and takes scope out of a relative clause. It is proposed that preverbal weiyi is part of a DP which undergoes overt A'-movement in the process of relativization. The syntactic structure and semantic composition of a matching analysis are offered to show how a uniform account can be given across adjectival and relative weiyi. A major implication of this paper is that Mandarin does not possess a relative pronoun but allows a DP-internal focus expression to mark syntactic movement in relativization. A comparison between relative constructions involving weiyi and English all-clefts is also discussed.