Rational number knowledge is an important factor in students’ mathematical development. However, many students face difficulties with rational number concepts. This study describes a new educational game, NanoRoboMath, which has been designed to support students’ rational number knowledge. Our goal is to examine the design and the nature of the mathematical practice in the game. Four comprehensive school students aged between 11 and 13 years individually played a prototype version of the game. Video recordings and log data of these game sessions revealed that the game was able to elicit mathematical activities related to rational numbers. This version of the game seemed to enable a variety of strategies matching different skill levels and supported arithmetic activities related to different aspects of rational number conceptual knowledge.