This paper attempts to measure the similarity of frequently occurring modal auxiliaries in both L1 and L2 writings. The modal auxiliaries are known to be difficult areas of study for both L1 and L2, due to the overlapping in their use. A subset of TOEFL11 corpus and a subset of PTB(Penn Treebank) corpus were used to capture the similarities among modal auxiliaries within L1 and L2, respectively, and also across L1 and L2. Based on the hypothesis of distributional representation, similarities of modal auxiliaries were computed by applying cosine similarity and mutual information to every pair of words in the corpus, and then finally reducing dimensions with Singular Value Decomposition. The results show that the modals are found to be similar to other modals, and the distribution of modals in the reduced dimensions show that the modals in L1 and those in L2 exhibits different patters of similarities, while the distance among the modals in L1 is further away than the distance among those L2 modals.