The aim of the present study, which was conducted using a descriptive exploratory method, is to investigate the understanding and misunderstanding of the concept of probability among eighth grade students. The statistical population of the study included eighth grade male and female students in Tehran province. A random sample of 1330 students studying in regular, government, control, and gifted schools was classified using a random sampling method and 15 probability questions, the validity of which was reviewed by a number of mathematics and mathematics education professors and teachers with mathematics experience, were asked. After conducting descriptive statistical analyses, students' misunderstandings were identified in seven groups as follows: lack of understanding of rational numbers and their relationship with fractions, inability to count all possible cases, mental judgments, language problems, use of self-made methods in calculating probability, inappropriate generalization, and lack of understanding of some prerequisite concepts.