Space environment naturally can damage electronics through two failure mechanisms, totalionizing-dose and single-event effects (SEE). In this presentation, a reliability quantification method is proposed by considering both failure mechanisms, The reliability of an electronic device due to the first failure mechanism is defined as the probability of the cumulative effect of the particles greater than an allowable threshold. The reliability due to the second failure mechanisms is defined as the probability of a high-energy particle impacting the device and causing the hard failure of the electronics. The reliability of the entire device is obtained by using Baye's theory. A cascade model for the cumulative deposited energy of Galactic ions is presented. Finally, a comprehensive example is given to illustrate the usage of the proposed method.