The Study was performed on a high-energy linear accelerator (Elekta PreciseTM). with built-in multileaf collimator (MLC) which produces two nominal photon energies (6MVand 15MV).Dosimetricmeasurementswere carried out using linear accelerator (Elekta PreciseTM), PTW-UNIDOS Electrometer, a dual (0.125 cc) ionChamber and Farmer (0.6 cc) ionChamber, MEPHYSTO Version 7.3, and Therapy BeamAnalyzer (MP3-S). Threedimensional treatment planning (Precise PLAN) was used for calculating dose distribution. In this study, the physical properties characterizing high energy photon beams of (6MV and 15MV) that include the central axis percentage depth dose (CADD), beam profile specification, tissue maximumratio (TMR), surface dose, buildup region and output factor, for different field sizes at different depths, were determined to showthe effect of field size and beam energy on the central axis percentage depth dose also to show the effect of field size and depth on beam flatness, beam symmetry, penumbra, TMR and out put factor. Also in this study, the physical properties characterizing high energy photon beams of (6MV and 15MV) were measured and compared with the corresponding published data and calculated data by Precise PLAN (3DTPS).