Prior research on the role of user participation in system development has yielded inconsistent results. One of reasons might be attributed to the various users and research in this field needs to take into consideration of the characteristics of users themselves. Users' professional background has been documented as one of major reasons for resistance. However, very little research has investigated how to resolve such resistance. Our study makes a significant step forward to understand and empirically validate the contingencies of business professional users' participation. With an exploratory case study, we identify three important contingencies that determine the effectiveness of professional users' participation in IT projects in terms of assimilation, i.e., IT competence, work-identity integrity and alignment as perceived by the professionals. The resulting model is also empirically validated involving professional users from different industries and organizations.