In this paper, a mathematical model for determining the reliability of ventilation plants was given and said ventilation plant was checked using a specific example for two types of fans operating in different modes due to the difference in the age of the fans. Within the framework of the paper, the principles of arrangement of ventilation systems of mines are stated, after which the mathematical model for determining the probability of reliability of ventilation systems is presented. For checking and proving the given mathematical model, coal mines in Serbia have been selected. The experimental part presents the basic characteristics of the main fan plants of active underground coal mines in Serbia, and based on the given mathematical model and collected data on downtime in the operation of the main fan plants at coal mines, the reliability of their operation is calculated. The paper confirms that the proposed method of determining the mathematical probability of reliability of ventilation systems can be used as an initial indicator of ventilation systems, where regular plant maintenance and application of this mathematical model could predict and prevent the operation of ventilation systems, and thus extend their lifespan. By processing the data on the operation of ventilation plants, the appropriate empirical dependence of the reliability parameters was obtained with the corresponding dependence, which enables application in practice.