In this paper, I suggest that we need contrastive studies through building parallel corpora in order to fully describe the differences of vocabularies of Korean and Japanese. And I show some researches dealing with actually built parallel corpora. I demonstrate that these researches are possible only through parallel corpora or they show that the research method of using parallel corpora is an efficient one. Furthermore, I argue that in order to expand the purview of the comparative studies of Korean and Japanese modern vocabularies and its method, we need to build not just parallel corpora for sentence alignment, but also parallel corpora for word alignment, and also we need to solve many problems including developing morphological analyzer of modern Korean.