The purpose of this study was to investigate the continuity of reading passages of Korean high school English textbooks with the 2015 Revised National Curriculum using CohMetrix, a software that analyzes English texts in a range of linguistic features. Twentyone English textbooks were analyzed: 11 High School English, five High School English I, and five High School English II. Only reading passages in textbooks were analyzed. Coh-Metrix indices related to basic counts, lexical features, syntactic features, cohesion, and readability were analyzed. Results revealed significant differences between High School English and High School English I and II, but no such differences exist between High School English I and II. This study implies that continuity of reading difficulty in terms of lexical, syntactic, and readability characteristics among High School English, English I, and English II textbooks should be considered.