This study aims at investigating the relationship between student's second language lexical fluency and their C-test performance. Using cluster sampling procedure,75 undergraduate students of Engineering and Basic Sciences studying general English at Shahrood University of Technology were randomly selected to participate in this study. Controlled Oral Word Association Test (COWAT) and C-test were then administered to gather data related to students' lexical fluency and C-test performance respectively. Data were analyzed through correlational analysis and independent-sample T-test. The results revealed a strong positive correlation of r= 0.79 between students' performance on C-test and their lexical fluency. In addition, it was revealed that lexical fluency predicts 54% of the variance in C-test performance. These findings have clear implications for instructed second language learning.