abstract this quasi-experimental study was aimed at examining the impact of storyline complexity on the grammatical accuracy and complexity of advanced and intermediate efl learners. a total of 65 advanced and intermediate efl learners were selected from iran language institute (ili). an intact group including 35 intermediate participants and another intact group with 30 advanced participants were used as the source of data collection. a paper-based toefl test was selected from toefl kit (2003) to check the proficiency level of the participants. having substantiated the proficiency levels of the two intact groups, the researcher gave two picture-based written narrative task with different storyline complexities. the distinguishing feature of the two given tasks was storyline complexity which was operationalized as +/- background knowledge and +/- foreground knowledge. the first written narrative task included only foreground knowledge (non-detailed picture prompts) but the second written narrative task included both foreground and background knowledge (more detailed picture prompts). both within-group and between-group comparisons were used to examine the impact of storyline complexity which was manipulated in the two versions of the same written narrative tasks. the results of the study indicated that complex storyline (+foreground and + background information) significantly improved the grammatical complexity of both advanced and intermediate participants of the study. however, storyline complexity failed to enhance the grammatical accuracy of the participants’ task performance within both groups. it should be noted that storyline complexity did not have differential effects on advanced and intermediate participants. that is, proficiency level of the participants did not moderate the impact of storyline complexity on the dependent variables of the study (complexity and accuracy). the findings of the present study indicated a remarkable effect of storyline complexity on grammatical complexity of written narrative tasks. based on the results of the study, it can be argued that narrative tasks can be used in efl classroom contexts as a device for promoting l2 language performance and language acquisition.