Since the advent of the Internet as a means of communication, more and more people have been using electronic media for a variety of purposes including interpersonal communication, sending and receiving information as well as language learning purposes. Therefore, a line of research in this area seems necessary to gain a better understanding of various aspects involved in this topic and identify its possible advantages. To this end, the present study was an attempt to investigate the effectiveness of an online messaging application, namely WhatsApp Messenger, which is an example of computer-mediated communication tool where both synchronous and asynchronous online communication can occur, on Iranian intermediate EFL learners’ syntactic complexity. This research was an experimental study and had a pretest-posttest design. This study had an experimental and a control group. The participants in the experimental group of this study were 15 adult intermediate language learners. They were asked to participate in eight discussion tasks and post on WhatsApp Messenger for a period of two months. The quantitative analyses were carried out by running independent samples t-tests as well as Mann-Whitney U tests to determine the differences in terms of syntactic complexity. The results revealed that using WhatsApp Messenger for communication practice had significant impacts for this group of learners on eleven measures of syntactic complexity. In addition, a survey questionnaire was administered to further delve into the participants’ perceptions and beliefs about their experience of computer-mediated communication. The results showed that participants found this messaging tool helpful and were satisfied with it in terms of comfort level. Participants also showed positive feedback toward the design and implementation of the study. / Graduate