The treatment of moving targets is a primary challenge of radiotherapy. Internal margin has been widely used to account for the geometric displacement of moving targets. However, it has not been fully addressed if internal margin is beam specific, rather than purely geometric. A tissue equivalent thorax phantom with a tumor placed in the right lung is used in our study. The simulated tumor moves in three directions, superior-inferior (SI), anterior-posterior(AP), right-left (RL) with 6 and 12 mm displacements. Thedose distribution was recalculated for each position simulated. The resultant dose distribution is analyzed to provideinsights of internal margin setup. Our results show thatthe internal motion induced dose error is considerably lessalong the beam axis than it is laterally. Therefore, the beam direction needs to be considered when the internal margin is used to account for the tumor motion in the treatment of lung cancer with proton therapy.