the ability of composing a coherent and extended piece of writing in second language is considered as a fundamental factor to convey information and ideas of learners through the academic issues. although learners may achieve a perfect academic writing skill through assigning the l2 tasks in content based instruction, but demonstration of their abilities may be related to their ability in l1 essay writing and their level of autonomy. the purpose of the present study was investigating the relationship among efl learners’ autonomy, first language essay writing and second language essay writing in task/content based instruction. to this aim first, 145 efl university students (99 female and 46male) of teaching and translation english of two branches of islamic azad universities, were selected based on of their performance on a sample of piloted toefl test and a sample test of written english (twe) from among 210 participants at advanced level. subsequently, a piloted autonomy questionnaire and eight l1 and l2 essay writing tests were administered (four tests of essay writing in l2 and four tests of essay writing in l1). correlation and regression analyses were used to analyze the data. consequently, the results of this research revealed that, there is a positive and significant relationship among efl learners’ autonomy, first language essay writing and second language essay writing in task/content based instruction. also it was shown in the data analysis that autonomy was a better predictor of english essay writing as compared to persian essay writing.