Inspired by current issues in lexical depth and previous extensive reading (ER) investigations, this study examined the effectiveness of ER approach in lexical depth of young EFL students in Iran. In the present study, two male classes were formed. One of these two classes was randomly selected as a treatment group and another one as a control group. The treatment group was provided with vocabulary instruction through the ER approach. However, the control group was taught traditionally and without the ER approach. Students in treatment group were provided with ER approach for 17 weeks. At the end of the study, a test of lexical depth was administered to both groups and a t test was used to compare means of test scores between groups. The results indicated that ER is an effective approach to improve students' lexical depth.