Modern ventricular assist devices (VAD), the devices for mechanical replacement of the cardiac function represent complex biotechnological systems. In addition to implantable parts, such system includes the peripherals, which are represented by various electronic modules, designed to provide precise control, management and uninterrupted operation of VAD. Therefore, the durability of the device is determined by the reliability of the VAD modules and elements. One of the major tasks during the development of VAD is an increase of reliability in these devices for the mechanical replacement of cardiac function. Herein, in the work the architectural designing of three versions of VAD has been performed. The structural circuit has been developed and the calculation of reliability for the given data of the architectural designs has been performed. The mathematical model of reliability in the form of a graph has been built and the system of differential equations for probability states has been solved. The calculation of the operation trouble-free probability for five particular cases of the architectural designs depending on the complexity of the technical cases has been performed. The work results have shown that with an increase of the complexity of the technical solutions the decrease of the probability of the trouble-free operation has been observed, as the probability of failure-free operation of the given system elements increases. The reliability in VAD depends on the probability of the failure-free operation of the system elements without possibility of reservation. An increase of the VAD architectural solution has been achieved by the duplication of the main functional blocks and by the decrease of the system complexity of the elements without reservation.