Abstract-This paper mathematically models the operation of Arrayed Waveguide Grating (AWG) based multiplexer (mux) and demultiplexer (demux) used in optical networks. In WDM networks, the optical mux and demux play a crucial role of managing the aggregation and segregation of wavelengths for networking applications. A simple and intuitive model of AWG based mux design is discussed in this work. This model assumes that the device is linear, in which the principle of superposition is valid, and the primary emphasis is given to the optical power gain of the individual wavelengths. By using this model, one can exactly estimate the individual and overall power associated with each of the multiplexed wavelengths. The developed model was evaluated with experimental results using AWG based multiplexers. The experiments were repeated for different test cases with various power input levels and multiplexer configurations. It was found that the developed model provided a good approximation to the actual AWG mux/demux.