In the present study, a quasi-experimental pre-post test design was used to assess the effects of an argumentative writing strategy (POW+TREE) on the performance of grade five and six students of Greek origin who were learning English as a foreign language (EFL) in a Greek setting. The Self-Regulated Strategy Development (SRSD) cognitive apprenticeship model was utilized to improve the text composition skills of the students. In the experimental group (N=77), participants received instruction on general and genre-specific strategy use for planning and writing argumentative essays, on procedures to apply self-regulation (goal setting, self-monitoring, self- reinforcement, and self-instructions), and on establishing additional skills (vocabulary, grammar-drill instruction, good word choice, interesting openings etc.). The control group (N=100) was supported through a traditional curriculum in writing (focusing on spelling and grammar). Findings of the study showed that strategy instructed students wrote argumentative essays that were schematically stronger, qualitatively better, and longer than those produced by their counterparts in the control group.