—By taking advantage of the multiple wavelength provisioning capability, WDM PON dramatically enlarges its capacity as compared to TDM PON. The capacity of a WDM PON system depends on many factors, such as the number and capacities of wavelength channels, the network architecture, the wavelength support of receivers, and the tuning ability of lasers. This paper introduces the definition of achievable rate region to describe the capacity of WDM PON and analyzes the achievable rate region for a given network architecture from the perspective of wavelength sharing. This can facilitate comparison of different WDM PON architectures and help design an efficient access control scheme.