The purpose of this paper is to evaluate vocabulary size gain by students using extensive reading (ER) through xReading, an online library coupled with a quiz program designed to motivate, verify, and track students’ ER achievements. The participants were 63 first year Japanese university students enrolled in a basic compulsory English course where the Start with Simple Stories (SSS) and Sustained Silent Reading (SSR) approaches were implemented. The participants had to take a 4-level Vocabulary test measuring the vocabulary size gain using a bilingual Vocabulary Level Test at the beginning and at the end of the first and second semesters. This test divides words into different levels based on their frequency and is most suitable for measuring vocabulary size and vocabulary knowledge of elementary and intermediate learners of English as a foreign language. It was first designed by Paul Nation (Nation 1993) and later adapted by McLean, S., & Kramer, B. By the end of the academic year, results show a significant vocabulary size gain especially in the more advanced terms.