Abstract-Maintaining wellbore stability is challenging in any drilling situation, especially when directional drilling with narrow pressure window are experienced. An imperative parameter to control wellbore stability is downhole pressure or equivalent circulating density (ECD). An accurate downhole pressure is required in order to maintain it in pressure window and also avoid drilling problems which cause interruption during drilling operation, resulting in high non-productive time. Since annular frictional pressure loss increases ECD, it becomes very challenging to estimate accurate annular pressure loss. Many experimental studies have been developed annular pressure loss prediction without validating results with field measurements. This study aims to estimate an annular pressure loss in directional drilling with or without pipe rotation using several developed models with casing program. The performance of the models are tested by comparing the results with field measurements obtained from Kam Phaeng San Basin, Thailand. The conventional annular frictional pressure loss combined with increasing-pressure-loss model gives a good agreement with field measurements, a pipe rotation effect is more influential on annular pressure loss especially in smaller annular space. In addition, a user-friendly software is also developed using MATLAB platform to predict real time downhole pressure and ECD with casing program.Index Terms-Downhole pressure, annular pressure loss, equivalent circulating density (ECD), directional drilling.