There has always been a debate whether or not the learners' first language (L1) can facilitate the process of learning foreign language. Since foreign language writing (FL) is a complicated process, it seems that the role of the learners' L1 and its effect on FL writing is of great importance in this regard. The present study aimed at investigating the role of Azerbaijani EFL learners' L1 and L2 (Persian) on their FL writing. To meet the purposes of the study, 30 female EFL upper-intermediate students were selected (through running an OPT), who were native speakers of Azerbaijani, with Persian as their L2 and official language of Iranian context. The data were collected through running three writing sessions (in which the participants wrote three essays in each session using Azerbaijani as L1, Persian as L2, and direct writing) using the think-aloud protocol, through which they were asked to report their thoughts loudly to record using a tape recorder. In addition, a survey was used to ask their perceptions towards each writing task. The analysis of the data obtained from the evaluation of learners' writings indicated that the mere use of their L1 or L2 in foreign language writing was by no means helpful for them and they performed better on direct writing task in comparison with the two other ones. It was also found that the majority of the learners (70%) had difficulty in generating their ideas using Azerbaijani as their L1. In addition, nearly 77% of them claimed that even in direct writing mode, they made use of their L2 (Persian) on the occasions they could not find a proper word or phrase in English. As the implications of the study, it seems that the present study can bring helpful insights for both FL teachers and learners about the roles that Azerbaijani and Persian languages play as the students' L1 and L2 in FL writing.