WAS (Web application server) is a distributed system which provides runtime integrated services for transactional Web applications. To improve the dependability and scalability of WASs, one effective way is to balance loads via adaptive load balancing service based on the middleware. The existing adaptive load balancing services, however, have inadequacies such as lack of server transparency and extensible load balancing strategy etc., and provide insufficient functionality to satisfy WASs. In this paper, the key requirements of adaptive load balancing services for WASs are addressed first, and then the design of a novel adaptive load balancing service is presented. The key design challenges including hot plug-in, customizable load balancing strategy, load feedback and adaptive control, state migration and fault tolerance etc. are described, and the technical solutions are outlined. Lastly, a comparative study with other related works is given The adaptive load balancing service is developed and implemented in WebFrame2.0 application server by using Java RMI.