Abstract:Abstract: The study aimed to examine the effect of task instruction and working memory on the written language production of Chinese learners of English by treating task instruction and working memory as independent variables and learners’ written language production (fluency, accuracy and complexity) as dependent variables. The results showed: 1) task instruction and working memory exerted no significant impact on fluency and lexical complexity of the written language production of Chinese learners of English; 2) task instruction had no significant effect on accuracy while working memory affected accuracy obviously; 3)task instruction and working memory had a significant effect on syntactic complexity; 4) the effect of task instruction on the written language production might be under the constraint of working memory which interacted with task instruction. Based on the aforementioned results, it is assumed that in teaching writing, task instruction should be clear to students and an appropriate task instruction should be chosen according to working memory capacity because both of them might affect learners’ writing performance.