Abstract: As the top level of management and IS managers spent too much of their budget in IT development, the assessment of WBIS is greatly needed in order to justify the IT investment in terms of its impacts on employee performance. In this concern, it is important to highlight that a very little research has been conducted to assess the two critical factors including interaction design quality and contextual performance. For this purpose, this study is primarily concerned with the development and validation of new multi-dimensional instrument and model for assessing the effectiveness of WBMIS. The effectiveness is assessed based on employee performance: Task performance and contextual performance. A sample of a total of 384 UNRWA respondents was gathered. The results of validity and reliability analysis provide enough confidence to proceed toward validating the developed assessment model. The results indicate that there is a positive significant relationship between information, service, communication quality, user satisfaction and effectiveness. However, the results showed that interface design and system quality have insignificant relationship with satisfaction. The theoretical and practical implications and future point of research are discussed. Finally, the empirically validated instrument and model would be helpful and contribute to the IS empirical studies.