In extensive reading (ER), Graded Readers (GRs), books for English learners with vocabulary control, play a central role whereas Leveled Readers (LRs), books written primarily for English-speaking children, are not always recommended due to their complex texts and low frequency words. In Japan, however, learners usually use a mixture of GRs and LRs to fill the gap below the easiest level of GRs. This study created the GRs corpus and the LRs corpus from books in popular series that are used in the initial stages of ER to explore the differences in grammar and vocabulary levels. Each corpus was composed of approximately 130,000 words. The New Word Level Checker was used to obtain vocabulary levels, and the Flesh Reading Ease was used to examine words per sentence and the percentage of passive sentences. The differences suggested that LRs were generally more difficult in grammar and vocabulary than GRs even though the same readability levels (YL) were assigned.