Abstract -  Enterprise Resource Planning (ERP) system is a complex software system that supports network of different business processes. Sales of such systems has expanded in the late 90s. In that period ERP installations dominated in large business organizations, but today are commonplace in small and medium-sized enterprises. It's very important for ERP buyers to check is it possible to adapt preferred ERP package to their business. ERP implementer can modify the software to fit the customer’s business process but that can slow down the project and introduce new defects into the system. It would be useful to gather input about the perceived defects during systems usage and to try to assess what a customer can expect in the future. We assumed that suitable models in modeling of ERP system reliability are Software Reliability Growth Models (SRGM). This paper presents how optimal selection of input parameters can be done during a system usage in an attempt to choose the appropriate SGRM model. In presented case study Weibull model is identified and used for reliability modeling and reliability prediction for existing ERP installation.