Studying explicit vocabulary instruction effects on improving L2 learners' writing skill and their short and long-term retention is the purpose of the present study. To achieve the mentioned goal, a fill-in-the-blank test including 36 single words and 60 lexical phrases were administrated to 30 female upper-intermediate EFL learners. The EFL participants were asked to write a composition titled "A Cruel Sport" after a reading activity on "Bull Fighting". Comparing this writing to the one written after target vocabulary instruction, it caused a significant increase in the number of vocabularies used productively in learners' writing. The statistical analysis revealed that in delayed writing, the participant retained the newly-learned vocabularies even sometimes after the instruction. Based on the obtained results, this research offers below suggestions for L2 instructors: (1) productive use of words is not guaranteed by word comprehension per se; (2) learners are not only able to increase the active vocabulary under their control but also use the words they just learned; and (3) in a writing task which was immediately fulfilled through explicit vocabulary instruction, vocabulary recognition is converted into a productive one, improving retention and leading to productive use of newly learned vocabulary at the same time. This productiveness, however, is loss prone and more practice is needed in producing newly learned vocabulary.