While the effects of L1 on L2 have been extensively investigated, the effects of L2 on L1 have been largely ignored. The present study attempted to address this issue by investigating the effects of L2 English on Iranian Bilinguals' L1 writing ability. For this end, 61 participants, 30 bilinguals and 31 monolinguals, were assessed on an essay-writing test in their L1. The gathered data were analyzed using independent samples t-test in which the t[subscript obs] (2.37) was higher than the critical value (2.00) at the significance level of 0.05. The bilinguals, as indicated by the results of the independent samples t-test, performed better than the monolinguals on their L1 writing ability indicating the positive effects bilingualism, here English, could have even on L1 writing ability, which provides more evidence for cross-linguistic influence.