In this study, it was aimed to reveal the ideas of the secondary sixth and eighth grade students about the concept of decimal via mental images. Moreover, within the scope of this study, the differences between the mental images of the sixth and eighth grade students about the concept of decimal number will be investigated according to their class levels. For these purposes, this study was carried out with a total of 212 students studying in the sixth and eighth classes of a state school in Bursa in the second semester of the 2016-2017 educational year. The research data was obtained as a result of the students' completing the statement of "A decimal number looks like a/an … because it …" As a result of the analyses, it was observed that the participant sixth and eighth grade students created a total of 162 valid mental images in relation to the concept of decimal. From these analyses, it was understood that the students perceived the decimal as equivalent to such different concepts as ball, pastry, money, etc. The mental images created by the participant sixth and eighth grade students in relation to the concept of decimal were gathered and evaluated under the categories of "complexity," "roundability," "phased progressiveness," "additivity," "subtractability," "multipliability," "divisibility," "solvability" and "other" of decimals.