The study was conducted to see if the provision of implementing corpora on English relative clauses would prove useful for Iranian EFL learners or not. Two writing classes were held for the participants of intermediate level. A record of 15 writing samples produced by each participant was kept in the form of a portfolio. Participants' portfolios in both the experimental and the control groups were analyzed to spot patterned errors. Having diagnosed the errors, both groups were instructed on the use of English relative clauses. The participants in the experimental group were instructed using the corpora as printed materials. The control group, however, were instructed using explicit definition, discussion and exemplification. Then, both group participants' portfolios were returned and they were asked to self correct their misapplied relative clauses. Interestingly, both groups had improved significantly. Two Chi-square tests on the use of "that" and "which" were run before and after classroom procedures on both groups. The first one suggested similar performance for the two groups but the second one favored the experimental group. The study suggests that applying corpora is an effective way to make students aware of their errors which ultimately leads them to self correction.