This research aimed to improve written expression (composition) skills of twelve grade students of the Al-Mazar secondary school by using the Tamer-Aien intervention model. The intervention model was designed by using metacognitive strategic concept, techniques, symbols and diagrams. This study was conducted at AlMazar secondary school in Jordan. A total sample of 22 students‟ from the same grade was chosen for the study. The baseline assessment was carried out to explore the preintervention writing skill score of the students prior to the intervention cycle one. Later, intervention and writing skill support strategies were carried out for twelve weeks. In the end, post-test was carried out to explore the differences occurred in the writing score of the students as a post-intervention effect. The findings obtained from the quantitative data showed the improvement in the written expression skills of the students. It was observed that through taking the above-mentioned measures, the teachers can help students improve their writing skills. Data were analyzed using the independent Mann-Whitney U test followed by Wilcoxon Signed-Rank test. The results showed that there was a positive effect on the students writing performance from the pretest at the beginning of intervention program up to the posttest at the end of the intervention program. The findings have implications for pedagogy as well as for research.