The web services concept is rising as a standard to integrate heterogeneous systems in distributed environment based on XML messaging. Recently, for that reason, many companies are trying to use web services for their B2C and B2B business. In order to successfully introduce the web services, it is required to guarantee a differentiated quality of web services. However, most of the current web services cannot provide differentiated policies for requested messages and the existing web servers cannot consider appropriately the quality factors of web services in the service level agreement between provider and consumer. In this paper, we analyzed appropriate quality factors of web services in application level and suggested priority allocation strategy for processes dealing with messages of web services using those factors. This strategy can allocate dynamically a priority to satisfy the service level agreement.