The aim of the study is to determine mathematics teachers' decisions about dimensions of the geometric figures, criteria of dimension and consistency of decision-criteria. The research is a qualitative research and the model applied in the study is descriptive method on the basis of general scanning model. 15 mathematics teachers attended the research. Familiar geometric objects were given and asked to decide their dimensions. The question of "how do you decide the dimension of a geometric object" was used to determine their criteria of dimension. Descriptive analysis was conducted for this question. The results revealed that half of the mathematics teachers indicated a criterion. The criteria of area-volume and length-width-height were common used criteria. It was seen that their criteria and decisions weren't consistent due to lack of their knowledge of geometric object and concepts of area and volume.