Flattening filter free mode (FFF) has been introduced in radiotherapy during the past decades, however, not much has been reported on its radiobiological effect. The purpose of our study was to compare the radiobiological effects of flattening filter and flattening filter free photon beams on chromosomal aberrations in peripheral blood lymphocytes. In our study the blood of the same healthy donor was irradiated with linear accelerator using both conventional flattening filter (FF) and FFF photon beams at dose rate of 3.57-23.08 Gy/min, using 6 or 10 MV. The dose-response calibration curves for dicentric + ring chromosomes induced by irradiation were fitted with linear-quadratic model. CABAS (Chromosomal Aberration Calculation Software) was used to prepare the curves. The coefficients and equations of the curves were calculated and compared with the results of other authors. We found significant differences in the number of aberrations at different irradiation parameters. Based on our results, FFF mode has a 10-20% higher biological effect than FF mode. These results can be used during radiotherapy or to estimate the biological doses in case of an accidental exposure to radiation.