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The aim of this study, that was carried out in descriptive of the survey, is to assess the understanding and misunderstanding of the concept of probability eighth grade students. The samples of this study are, all eighth grade boy and girl students of Tehran province. The study sample,1330 eighth grade students in Tehran who were selected randomly. A random sample of 1330 students, from different public school. intelligence school, Shahed and perspicacious school randomly classified. and they were given 15 questions. which their validity has been studied by the number of math professors and teachers of mathematics and experienced math teachers. The reliability tests with Cronbach's alpha coefficien. 961 Was confirmed.After analyzing descriptive statistics, misunderstanding of the students were identified in seven groups as follow: lack of understanding rational numbers and its relationship to fractions, lack of understanding some of the concepts prerequisite, language problems, using their own methods to calculate the probability, inability to count all possible states, inappropiate generalization and the inability of the undrestanding of prerequisite problems.