With a paucity of longitudinal studies done in the stream of Technology Acceptance Model, a longitudinal case study research was conducted with a large state Owned enterprise (SOE) in Thailand that has gone through an ERP implementation project. To provide a more comprehensive perspective of the process of user acceptance of ERP, data were collected from two phases of the implementation process in order to capture the change of user attitudes and user expectation and examine how these two constructs are related to intention to system usage. This study presents a prospective combining view of Technology Acceptance Model (TAM) and the expectancy theory to examine how ERP users form their attitudes and expectations throughout the course of ERP implementation.