The paper presents an analytical model for a GSM-based cellular mobile network that applies the Intelligent Under-Overlay (IUO) scheme to increase the capacity by increasing the frequency reuse while maintaining the service quality. The IUO is a multi-layer cell structure that is based on dividing the frequency band into the super layer and the regular layer frequency group. The super frequencies (channels) can be used by mobile stations with good C/I (carrier/interferer) ratio, while the regular frequencies can be used over the whole cell. The use of IUO is expected to provide up to 40% gain of capacity [1]. In this paper, we study the effect of various parameters on the performance of the networks using IUO and provide practical planning support based on the analytical results. The considered parameters include network parameters like super area coverage and mobile user mobility parameters like moving mobile ratio and average mobile speed.