This study aimed at investigating the effects of an instructional unit based on a theoretical model of argumentation (Tolmin's ) on college students' writing performance, writing efficacy and writing apprehension. One hundred and forty three (143) second year college students in a faculty of education, English majors, were assigned randomly to one of two conditions. Both groups engaged in pretest and post-testst measures of writing performance, writing apprehension and writing self-efficacy. Control group participants were taught the regular writing course while the experimental group participants were taught the regular course in addition to the proposed instructional unit. The results of the study show that experimental group participants showed significant improvement in their argumentative writing performance. In addition, it was found that writing apprehension and writing self-efficacy have no influence over writing performance. Furthermore, the results suggest that writing self-efficacy is what influences writing apprehension and not vice versa.