The present study investigates four kinds of lexical diversity measurement and a computational experiment with corpus processing and statistical test has been conducted to find out the most effective lexical diversity measurement in evaluating a small-sized corpus of 350ź~ź550 words. The results show that the D-estimate is the most appropriate among the four lexical diversity measurements which were compared in this research. Also the D-estimate showed more stable results than other measurements when the number of words varied between texts. The D-estimate was applied to measure the morphological and grammatical diversities of L2 learners of the Korean language, and conduct a statistical test on whether the mother tongues of L2 learners affect the degree of acquisition of grammatical morphemes. The test shows that the native languages of L2 learners learning Korean did not seem to have a significant impact.