This study aimed at investigating the impact of extensive reading on improving reading proficiency. The study tried to find the effect of ER on EFL student's reading, vocabulary and grammar. The researcher designed two instruments; a program based on the extensive reading strategy and general test. Forty-one university students who study English was purposefully chosen from several universities in the academic year 2012 and the study continued for three semesters. To establish the validity for the instruments, the method of content validity was used; the instruments were given to a jury of specialists. In addition, the reliability of the test was established. Before carrying out the experiment, a pre-test for the general reading comprehension was administrated. By the end of the experiment, the researcher administrated the general reading comprehension post-test. The researcher used the t-test to detect any significant differences between the pre-test and post-test on the reading, grammar, and vocabulary. The findings show that ER improves university EFL students' reading, vocabulary and grammar achievement. Finally, the researcher suggested several recommendations.