This book is an explanation about the effect of task complexity on Iranian EFL learners' grammatical accuracy in written discourse through a study that was investigated by using a cartoon picture, and 60 intermediate learners that were selected from among learners in Kish institute in Sari - Iran.In this book first of all we survey the relation of writing and task based language teaching therefore we describe subjects about writing, tasks, task approaches, and task complexity.Language and writing or two distinct system of signs, the second exists for the sole purpose of representing the first the linguistic object is not both written and spoken form of words, the spoken forms alone constitute the objects. (sausure 1959: 23)