Aim of Study: The aim of this study is to report the variation of depth dose and surface dose of different field sizes by flattening filter free (FFF) photon beams generated by millennium True Beam medical linear accelerator used for cancer treatment and its variation with respect to flattened beams (FB). Materials and Methods: We compare the data of photon beam (beam characterization and evolution of dosimetric properties) for two standard FB photon energy 6 MV, 10 MV verses 6 MVFFF, and 10 MVFFF FFF beam. Percentage depth dose (PDD), surface dose and Quality index were the parameters analyzed for the comparison. Surface doses were obtained for field sizes 3 × 3 ‑ 40 × 40 cm2. For depth dose curves ionization chambers and radiation field analyser were used and data were taken at a source‑to‑surface distance (SSD). Consistency of data was checked for one year. Results: Surface doses were higher in FFF beam compared with FB up to 20 × 20 cm2 field size. But in higher field size 40x40 cm2 surface dose in both 6XFFF and 6 MV FB beams are equal. For 10 MV, surface dose is higher than 10XFFF beams in higher field size 40x40 cm2. Beam quality of FFF beam was lesser in magnitude in comparison to that of FF beam and the difference of 5.54%between 6XFFF and 6 MV, and 4.45% between 10XFFF and 10 MV were observed. For PDD at 10 cm depth, the maximum percentage of variation in 1 year for 6MV and 6XFFF beams with field sizes 10 × 10 cm2 and 20 × 20 cm2 were 0.50%, 0.54%, 0.81% and 0.58% respectively. Whereas for 10 MV and 10XFFF the maximum percentage variation were observed up to 0.35% within one year. Conclusion: The surface dose, PDD and Quality index consistency of FFF beams and FB photon beams showed good consistency during a year duration