The primary goal of this paper is to investigate whether a computer-based simulation can detect the difference between novice and expert teachers’ decision-making in mathematics instruction, which is complex in nature. The design of the simulation is grounded in a sociological perspective on practical rationality of mathematics teaching. The simulation consists of classroom scenarios, in the form of cartoon-based storyboards, with a series of decision moments to simulate the instructional situation of doing proofs in geometry. Empirical data helped verify and revise our design hypotheses/principles and showed that the simulation was able to detect some differences between novice-teacher and expert-teacher decision-making. Results of this study could inform the development of more advanced, computational models of mathematics teachers’ decision-making.