Imprecise propagation models lead to networks with high co-channel interference and a waste of power. In this study, we aim to adapt a propagation model for Nablus as we examine the applicability of propagation model in Nablus for GSM frequency band. The study helped to design better GSM network for the city area , In spite ofthe limitations of geographical terrains and frequency channels. The modification is accomplished by investigating the variation in path loss between the measured and predicted values, according to the propagation model for a specific cell BS 1. Then, more modification is performed on the model according to the results obtained through more investigation. We will then verify our modified model by applying it for other cells and conclude the results. Theoretical simulation by the model and the obtained experimental data is compared and analyzed further. Bertoni-Walfisch model -without tuning- give us the best results in mean error (1.426 dB), which is much less than mean error in Standard Macrocell model -used by JAWWAL-Palestinian mobile communication company- (10.91 dB) for BS 1 . These two models hadbeen tuned using Linear Least Square algorithm to fit measured data for GSM-900 in Nablus city, which is a vital step in the planning and rollout of wireless networks. Finally, to make sure that Bertoni-Walfisch is better than any other path loss model, we made a comparison between them in RMSE, Standard Deviation and Mean Error