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In oil and gas wells scenario, casing design is a crucial stage of the project, representing the major structural elements responsible for maintaining well integrity through its lifetime and allowing adequate production activities. The occurrence of failures in casing systems can lead to irreversible safety problems in well operation. It is estimated that between 85% to 95% of the failures in well tubulars are related to joints (Payne et al., 2006). This work aims to analyze mechanical models of API casing connections in a probabilistic framework. The formulation for evaluating the strength of tubulars, under various load conditions, is presented by the code API TR 5C3 (2008), widely used by oil and gas companies worldwide. The reliability analysis is performed by the Monte Carlo simulation and First Order Reliability Method (FORM), considering failure modes by fracture in round and buttress connections, beyond failures by pull-out in round connections.