This paper presents a Marginal Reliability Impact (MRI) based resource accreditation framework in the context of capacity market design. Under this framework, a resource is accredited based on its marginal impact on system reliability, thus aligning the resource’s capacity market value with its reliability contribution. A salient feature of the MRI-based accreditation is that each unit of accredited capacity from different resources will have the same reliability contribution, implying “substitutability” among supply quantities of capacity, the desired feature of a homogeneous product. Moreover, with MRI-based capacity demand, substitutability between capacity supply and demand is also achieved. As a result, a capacity market with the MRI-based capacity product can better characterize the underlying resource adequacy problem and lead to more efficient market outcomes.