This paper aims to reveal primary teachers knowledge of the quadrilaterals about the concept of quadrilaterals by examining the definitions of the quadrilaterals. A total of four primary teachers, 1 female and 3 male teachers, participated in the study which determined by maximum diversity sampling. The data collection tool consisted of four open-ended questionnaires, which were not based on mathematical procedural knowledge of the teachers and aims to reveal the subject matter knowledge of quadrilaterals. The first question is about the definition of the quadrilaterals (square, rectangle, trapezoid, parallelogram, rhombus and deltoid), the second question is the determination of the characteristics of the quadrilaterals, the 3rd question is the comparison of the characteristics of the quadrilaterals and the 4th question is about related to the nomenclature of the quadrilaterals. The data were analyzed by using descriptive analysis method (Zazkis & Leikin, 2008). As a result of analyzes, it was seen that the subject area information of the participants was insufficient. In order to be successful in the teaching of geometry, the in-service training activities should be organized in order to eliminate the deficiencies in the subject matter knowledge and the necessity of reviewing the mathematics courses they took at the university, which can be corrected first, in the context of the content.