The paper aims to make a comparison study between word association of native speakers and that of Chinese English learners (CELs). Through data analysis of the word association results, the nature of the second language (L2) mental lexicon is explored. A continuous free word association test (WAT) was conducted to 150 students from Dalian University of Technology (DUT). And the Minnesota word association norms are selected as a native speakers’ word association test for the comparison. The results of WATs are classified and analyzed with respect to response type and part of speech.The major findings in the paper are as follows: (1) The words in L2 mental lexicon are essentially semantically-related, just like the mental lexicon of L1 speake...