Due to the advance of technologies in multimedia compression and network communications, scalable media streaming services have been availed to provide QoS differentiated services for heterogeneous users. However, it is still a big challenge to support consistent end-to-end Quality of Services (QoS) for the users due to the dynamic feature of the Internet, and abrupt variability of the network resources may severally affect the client perceived QoS. In this paper, we address the issues of smooth QoS adaptation for scalable streaming services. We propose an Optimal Smooth QoS Adaptation (OS-QA) strategy which allocates the server resource adaptively to cope with the variability of network bandwidth and protects the service quality of different quality classes under dynamic resource constraints. We analyze the quality variation caused by resource fluctuation and proposed OS-QA to minimize the average QoS variance under the resource constraints. Simulations are conducted to compare our proposed method with other QoS adaptation methods, and performance is analyzed in terms of QoS variance and PSNR. Results show that our proposed method is able to gracefully adapt the QoS and protect the client perceived QoS by minimizing the QoS variance under dynamic network resource constrains.