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Background: The aim of this work was to study the output factors for Linac using different ionization chambers and build up caps. Materials and Methods: Output factors were measured for open square fields (3 × 3 cm to 40 × 40 cm) defined by collimator jaws for 6 and 15 MV photon beams from a Varian Clinac 2100C accelerator were measured. The measurements in air were performed using Compact Cylindrical and Farmer type ionization chamber fitted with acrylic and brass build up caps. All measurements were taken with the detector set with its central axis perpendicular to the beam central axis and isocentrically positioned at the reference depth in empty water phantom. Results and Discussion: It was observed that out put factors increased with field size for both 6 and 15 MV photon beams. The increase in output factor is less prominent for brass build-up caps than acrylic build up caps. Up to 1.53% and 0.97 % difference were observed between 6 and 15 MV energies for acrylic and brass build up caps respectively. For acrylic build up cap, no significant difference was found for both ionization chambers. However, measurements for brass and acrylic build up caps with same ion chamber differ by up to 4.4 % for 15 MV energy. Conclusions: The measurement of output factors Shwith cylindrical build-up cap made of high Z material distorts Shand should be avoided. The use of buildup caps with Zeff close to that of water such as acrylic is a good choice. Ionization chamber is the best choice for Sh measurements for large field sizes. Iran. J. Radiat. Res., 2012 10(2): 95‐98