The Internet contains many types of services with different security options, which may be updated very frequently. An entity selects only those services in trusted networks that satisfy its security requirements.The primary challenge is to determine the trust level of services in such networks, which contain devices with limited processing power, such as Internet of Things (IoT) devices.We propose a hybrid approach to formulate the trust of a security system in IoT networks. Our approach provides a systematic way to model the trust of security from the viewpoint of an entity. Furthermore, using a case study, we evaluate the approach via simulations of a smart entity that has to select a trusted network. The evaluation results show that our approach provides satisfactory and flexible trust computations.