The aim of the study was to develop a scale to determine number sense of middle grade students and through the developed scale to introduce a structure of number sense and make a definition reflecting its structural characteristics. Therefore, a draft scale was prepared based on the six dimensions identified by Yang (1995). The scale was applied to 584 middle grade students from four different schools located in the districts of Cankaya and Golbasi in Ankara. The Cronbach's alpha coefficient was found to be .86 for the scale with seventeen items. The result of the exploratory factor analysis showed that the scale consists of three dimensions. Eight items clustered in the first dimension, four items clustered in the second dimension and finally, five items clustered in the last dimension were defined as flexibility in calculation, conceptual thinking in fraction and using benchmark (reference points), respectively.