After the emergence of numerous Internet streaming applications, rate-distortion (R-D) modeling of scalable video encoders has become an important issue. In this paper, we examine the performance of existing R-D models in scalable coders by using the example of MPEG-4 FGS and PFGS, and propose a novel R-D model based on approximation theory. Experimental results demonstrate that the proposed model is very accurate and significantly outperforms those in previous work.