In 1988 a private data mining firm, MetaMetric, created Lexile index. It is different from any other readability index in that it measures both students’ comprehension capability and reading material difficulty to read. Since its publication, Lexile index has been prevailing in U.S. society to the extent that almost half school children areevaluated their reading comprehension capability with the index. Furthermore, Common Core State Standards, an education policy given by Federal Government, consequently promotes the use of Lexile index to improve students’ reading capability. 150 publishers including McGrow-Hill and distributors such as amazon.com also facilitate the index in their business. The influence of Lexile Index is now covering U.S. education entities from K to 12. This paper discusses the general concept of readability index first. Then, it shows how to calculate a basic readability index. Finally, the methodology of Lexile Index is described and discussed.