Introduction: error occurrence in computer systems, can lead to irreparable damage, especially those used in medical systems. As a result, verification of such systems is important. Model checking as a method is used to ensure the absence of errors in the model. The heart-lung machine is used in surgeries in which heart must stop working and assumes the heart and lungs duties. In this article, a formal approach is to verify the operation of the heart-lung machine.
Methods: The heart-lung machine has been modeled by using the UPPAAL tool which supports time automatic machine, since, this machine do three sets operations in parallel which has been modeled in three subsystems: system overall performance machine, heparin injection machine and cardioplegia solution delivery machine.
 Finding: After modeling by a complete search on state space of model, the most important characteristics of system were verified. Situations were identified which cause entering the system to unsecure states. The reachability of all important states of the system was investigated. Finally, we ensured about system accuracy features and the system operates correctly.
Conclusion: Modelling is a cheap way to study a system and evaluate its reaction to environmental changes before implementation of the system. Considering to importance of heart-lung machine in surgeries, in this research a formal model has been presented to verify the operation of this machine.