The purpose of this study is to develop a Computer Adaptive Testing (CAT) smart app in elementary English reading education for extensive reading that gives students their readability index by means of a test before elementary school students began their reading. Compared with the web based Korean elementary English readability index (KEERI), software developed by Kim and Moon (2012), the CAT smart app tends to be a pre-test. And it is based on smart devices and CAT. Based on 700 readability index evaluation questions, the CAT smart app has been developed to measure students’ readability index. The core engine used in the CAT smart app was implemented in PHP, Java SDK and Android SDK. After analysis of a survey conducted by expert group, the need for CAT was recognized. So the CAT system on the web was developed, too. After the development of the CAT smart app, a correlation analysis was conducted with the students’ evaluated readability index and English reading ability performance assessment, and it was found that there was a high correlation. It showed that the students who had a high readability index got high scores in their English reading ability performance assessment and the higher the students’ score, the higher the correlation was.