Most of the studies on the key word method of second/foreign language vocabulary learning have been based on the evidence from laboratory experiments and have primarily involved the use of English key words to learn the vocabularies of other languages. Furthermore, comparatively quite limited number of such studies is done in authentic classroom contexts. The present study inquired into the effect of using mnemonic key word method of vocabulary instruction on the learning and retention of vocabulary over long term in a normal EFL classroom context.Fifty5th grade primary school students were selected and randomly assigned into experimental and control groups. The experimental group received vocabulary instruction using mnemonic key word method and the control group received classic memorization based instruction of the same vocabulary items. The two groups took three posttests a day, two weeks, and a month after the last treatment session. A MANOVA analysis was run on the data and the results indicated that subjects in the key word group outperformed the memorization group at a significant level in both their learning and retention of the newly learnt vocabularies. The results of the study underscore the efficacy of the establishment of mental links and images for the vocabulary learning and retention of novice and beginning level EFL learners. It further implies that mnemonic devices like key word method should be given prompt attention by both EFL material developers and practitioners as a potentially effective strategy for vocabulary teaching, learning and long term retention at the early stages of second or foreign language development.