While there has been ample research on the relationship between vocabulary knowledge (breadth and depth aspects) and speaking proficiency, very little attention has been given to the correlation between the four aspects of L2 lexical knowledge (receptive/productive/breadth/depth) and L2 speaking ability, and none has focused on the moderation effect of L2 vocabulary fluency in speaking capability. The current research recruited 312 Chinese university students as participants. It used the Structure Equation Model (SEM) to assess the relative effectiveness of the four aspects of L2 lexical knowledge and the predictive power of the overall L2 lexical knowledge in L2 speaking. It also ascertained the moderation effect of L2 vocabulary fluency by using Ping’s Single Product Indication Method. Results showed that of the four aspects of L2 vocabulary knowledge, productive vocabulary depth contributed the most to speaking skills. The productive vocabulary breadth ranked second, with receptive vocabulary breadth and depth showing less contribution to speaking proficiency. Regression analyses indicated that overall L2 lexical knowledge explained 47% of the variance in speaking. On the basis of these research findings, educational implications are further discussed, then limitations are identified.

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