Vocabulary learning enjoys a paramount significance in language learning. As such, vocabulary instruction gains importance as well. Based on studies in cognitive psychology, linguistics and new developments in vocabulary instructions, the current study deals with two methods of vocabulary instruction: lexical sets (LS) vs. semantically-unrelated (SU) words. In the first method, words belonging to the same "lexical field/semantic domain" were presented together and under particular topics. In the semantically-unrelated method, however, words were presented sporadically and isolated from other members of the same lexical set. The purpose of the study was twofold: (a) to assess the effectiveness of the LS and SU vocabulary instruction, separately and relative to each other, and (b) to assess the differential effects of the two instructional methods for the students of lower and upper English proficiency levels. The two groups of learners were taught the same vocabulary items. Vocabulary knowledge scale (VKS) was used to measure the learner's breadth and depth of knowledge of target words. The results of the analysis indicated that the learners in the LS method made significantly greater gains in their vocabulary depth (VD) and vocabulary breadth (VB) than those in SU method. However, the upper level learners consistently showed greater gains than their peer lower level learners.