Abstract: Electronic English Vocabulary Size Evaluation System for Chinese EFL Learners constructs a word frequency table from the British National Corpus, and randomly extracts sample words from the word frequency table to construct productive and identification items. With the data from over 1,000 test takers, the modern test theory, Item Response Theory, is introduced to carry out the model fit test. The model fit probability value is taken as the standard to pick out items. Simultaneously, the three major parameters of the items are calculated. The qualified items are divided into ten grades and stored in the item bank. Then they are randomly extracted to form the test paper by applying the normal distribution theory. Finally, the confidence limit and interval estimation principles are used in the system to evaluate Chinese EFL learners' productive and identification English vocabulary sizes. Therefore, the system has a higher reliability, maneuverability and technicality.