The main target of this study was to test, as it is a quasi-experimental research by design, the effect of morphological awareness on vocabulary development of Nazret Debre Eyesus High School students (N=90). For this sake, it was intended to see the mean differences within and across groups during the two sets of tests- pre- and post- tests. Finally, this study aimed at justifying the correlation size of the effect of MA and vocabulary knowledge enhancement. Having been contained this major objective; two sections, out of eight sections, were selected and assigned in to control and experimental groups through lottery method. Each section normally had a total of 45 participants/learners. Treatment was given to the experimental group, but the control group was being taught with the usual classroom instruction. Tests were used before and after the intervention as a major data gathering instruments, and descriptive statistics, correlation, paired samples t-test analyses and independent samples t-test analyses were deployed in order to analyze the collected data. The descriptive statistics values revealed that there exist similarities and differences between pre-test mean scores and post-test mean scores of the groups respectively. Moreover, it was indicated that a quite strong correlation between morphological awareness and EFL learners’ vocabulary development existed. Vocabulary development was linearly positively correlated with morphological awareness. The paired samples t-test analyses results pointed out that there was important enough similarity between the control groups’ pre- and post- tests mean scores. However, it was observed that there was a significant difference between the pre and post test mean scores of the experimental group. These findings indicated that the difference and the similarity were due to the treatment. Finally, the independent samples t-test analyses results, in the same way, pointed out that there existed similarity and difference. The pre-tests mean scores of both groups, for example, revealed that there was no significant difference between them, meaning that there existed strong similarity between mean scores of both groups in their pre-tests. In contrast, the independent samples t-test analyses results of post-tests of both groups indicated that significant difference between mean scores of groups was obtained. Therefore, the findings of this study, collectively, suggested that morphological awareness had significant and considerable positive influence on EFL learners’ vocabulary size enhancement since both had shown direct relationship to each other. Therefore, it is suggested that EFL teachers need to be involved in conducting/delivering a kind of morphological awareness creation classroom instructions so as to develop EFL learners’ vocabulary knowledge by using MA as VLS. Similarly, EFL material developers need to set clear objectives and contents in the materials for teachers to practice out the strategy in the classroom instructions.