L1 use is commonly viewed as predominantly debilitating to L2 learning. However, more recent bilingual researchers believe that L1 is not to be taken as a barrier, but as mental weaponry which can help control or regulate L2 learning processes (Cumming, 1989; Wang, 2003). To illuminate the authenticity of the recent conviction in bilingualism, the present study set out first to find out 1) if Iranian L2 learners involved in L2 composing task backtrack to rely on their L1; (2) how often they do so if any at all; (3) under what conditions they rely on their L1; (4) and finally what functions are likely to be served while doing so. To this end, the study used a think-aloud data collection procedure. The analysis of the results indicated that the subjects more often than not resorted to their L1 in order to function properly in the L2 composing task. The total number of L1 use was 1519 within 7535 words produced by the participants. As to the third question, the analysis showed that the subjects resorted to their L1 under the following conditions: (1) initiation; (2) mental investigation; (3) rectification; (4) rganization; (5) coherence construction; and (6) meaning confirmation. Their dependence on L1 while writing in L2 was taken to serve the following functions: (1) cognitive; (2) generative; (3) linguistic; (4) metacognitive; and finally (5) communicative. Overall, the results indicate strong positive role L1 plays in the EFL learners’ functioning in L2 and highlight the necessity to develop a theory of L2 writing, incorporating the unique features of L2 writing.