Abstract: The 2D ion-chamber array is widely used in dose distribution measurement in radiotherapy, and its effective point of measurement (EPOM), i.e. the accurate dose-space mapping position related to the dose from ion-chamber, is an important problem in accurate radiotherapy. In this study, Monte Carlo simulations were employed to research the EPOM for 2D ion-chamber array. Both the actual dose distribution along the central axis of photon beam in a water phantom in the absence of the probe and the dose curve from 2D ion-chamber array were simulated to locate the position of the EPOM by correlation analysis. It is found that the EPOM for 2D ion-chamber array is shifted from the front face of the ion-chamber cavity toward the cavity center by less than 1 mm. And the EPOM depends on the ion-chamber layout and the beam characteristics. The EPOM shift increases with the length of the cavity and the energy of the beam, and decreases with the increase of the cavity diameter.