Hidden failures of relay protection are the main cause of transmission lines' cascading trips. This paper defines the regions of vulnerability of hidden failures by analyzing the action mechanism of the hidden failures in protection equipment. A probabilistic model， for transmission lines'cascading trips， is established by calculating the probabilities of hidden failures of protection equipments and the fault probabilities of regions of vulnerability. This model considers the impact of transmission lines having different faulty probabilities on the system protection equipment. It can estimate the probability of cascading line trips caused by hidden faults in the relay protection system. The validity of the model is shown with an example. The results provide valuable information for power system reliability analysis and risk assessment.