The purpose of this study was to determine the effect of instruction of vocabulary breadth (size of vocabulary) and depth (morphology of words) on EFL learners’ vocabulary learning and reading comprehension. To achieve this purpose, vocabulary size test, morphological test and reading comprehension test were administered. All the tests were designed by the researchers in this study and they were selected from Select Reading and Active Reading books. To be valid and reliable, three tests were piloted on similar groups; Cronbach’s alpha reliability was used to estimate the reliability of the tests. The present study had a pretest posttest quasi-experimental design and it was conducted in intact classes. Seventy-five pre-intermediate level EFL students from Iran Language Institute were randomly assigned to one control and two experimental groups. All the students had the same language proficiency and were homogeneous groups. The control group did not receive the experimental treatment. A reading test and two vocabulary tests were administered to all three groups at the beginning of the study as pre-test. The students in breadth experimental group were given reading comprehension test, vocabulary size test and a glossary of words. The researcher went to this class three sessions as treatment and read the meaning of twenty words in each session, and then asked students the meaning of those words in English. The students in depth experimental group were given reading comprehension test vocabulary morphological test. The research taught different word classes for two sessions. Test of normality, test of homogeneity, one-way ANOVA and Pearson correlation determined the possible relationship between reading comprehension and vocabulary knowledge. This study implied that breadth and depth of vocabulary knowledge in reading comprehension assisted language learning. This study revealed that breadth of vocabulary instruction is better in improving EFL learners’ performance in re
