Corrosion Under Insulation (CUE) is found to be a major problem for insulated pipes as it becomes even worse as the pipes age increases. Gradual metal loss due to CUE may cause the pipe to leak or in the worst scenario, the pipe may rupture. The presence of small leak or rupture in lines may lead to a hazardous situation since the piping systems carry hydrocarbons or other process fluids. Hence, a proper inspection and maintenance program of these piping systems is crucial to maintain a safe and continuous operation. Corrosion and inspection engineers mostly are confronted with the problem of optimizing the inspection schedule for insulated piping systems due to the fact that most inspection/maintenance optimization models are based on the reliability models to predict the failure probability of the system. However, no reliability model was found within the literature that has currently been applied to insulated piping systems. This study presents a methodology for the estimation of the reliability of insulated piping systems containing corrosion defects. A probabilistic approach is adopted to this methodology and the associated variables are represented by normal probability distributions. First-order reliability method is employed for carrying out the reliability analysis due to the presence of nonlinearity in the limit-state function and also in the possible presence of non-normal variables. The methodology is applied to a real case study and the remaining useful life of the insulated pipe is assessed.