The present study sought to examine the continuity of the English textbooks of the elementary school 6th grade and the middle school 1st grade by analyzing the readability and vocabulary difficulty of their reading passages. It investigated the continuity between textbooks of the two grades and among chapters of each textbook. For the analysis, Flesch-Kincaid Grade Level Index was used to measure the readability of reading passages, whereas vocabulary difficulty was measured in terms of TTR (type-token ratio) and frequency by using VocabProfile. The results showed that the readability of middle school textbooks was one point higher than that of elementary school textbooks. Given that the readability index is based on the American school system, the increase in readability index between grades can be seen as large gap. In terms of vocabulary difficulty, the total amount of vocabulary in the middle school textbooks was three times as much as that of the elementary school textbooks. Second, readability index fluctuated across the chapters of each textbook, while the TTR was found to be higher in the former chapters than in the latter chapters in elementary school textbooks. All these could lead to learning difficulties for students. Pedagogical implications are discussed.