This study aimed to investigate the relationship between breadth and depth of vocabulary knowledge and reading comprehension of public secondary schools of Islamabad. The sample was based on 124 students; 72 male and 52 female from 12 public sector secondary schools. The students were selected conveniently from 10th grade with age range from 13 to 18 years, (M=15.10, SD = 0.998). They were taught English as a compulsory subject included in the school curriculum. The assessment of vocabulary knowledge and reading comprehension of students was measured through four research instruments; Vocabulary Level Test, Word Associate Test, Morphological Knowledge Test and Reading Comprehension Test. Results showed that a high correlation occurred between depth of vocabulary knowledge and reading comprehension, but a moderate correlation was observed between breadth of vocabulary knowledge and reading comprehension. Regression analysis showed that vocabulary depth has high predictive power than vocabulary breadth. The study advocated to mount students' depth of vocabulary knowledge to increase their reading comprehension abilities.