Objective: In this paper we scrutinize the concept of null curriculum critically—what schools do not teach—and two questions were in the focus: 1) what are different kinds of null curriculum? 2) What general factors affect the null curriculum in the Iranian educational system? In each of these questions, particular attention is given in clearly defining this almost ambiguous concept. It also defines two types of null curriculums namely intended and implemented null curriculum and the factors that affect the development of it. Background: Curriculum development has always been a matter of concern for educational institutions and it is one of the most important areas in language teaching and learning. The concept of curriculum can be recognized as a connective link between teacher and student, organized in such a way to achieve goals determined by the teacher already, the learning organization or by the curriculum designers. It must be noted that conception of curriculum plays a significant role in defining the null curriculum and pointed out that relationship to a given null curriculum can be identified only in relation to what is valued as educationally significant. Method: in this study, several studies related to curriculum designing as well as other aspects of curriculum were reviewed. Furthermore, we also searched various scientific sites to obtained related information and required data. Afterwards, theoretical views, problems involved and related strategies about them were presented. Conclusion: The tips and suggestions presented in this article can be applied in certain practical areas of curriculum development and evaluation. Curriculum and syllabus designing is one of the fundamental cornerstones of every educational system. The focus of this paper is on null curriculum, although various variables affect curriculum designing, the most important of all were presented here. It also reviews the role of teachers and the students in designing curriculum and investigates how important are their roles in establishing and implementing curriculum. It seems that there are some determining factors that affect curriculum and syllabus designing in the Iranian educational system and also some tips that syllabus designers and developers must take into consideration to meet the educational needs and purposes.