In this study, gamma ()-irradiated methyl 4-methyl benzoate (C9H10O2) single crystals were investigated and the structure of the free radical occurring by -irradiation was determined by using Electron paramagnetic resonance (EPR) technique. C9H10O2 single crystals were produced by slow evaporation with distilled water solution at room temperature. These single crystals were irradiated by Co-60-gamma-ray source with a dose speed of 0.981kGy/h at room temperature for 72h. The EPR spectra of irradiated C9H10O2 single crystals were recorded between 125 and 400K temperatures in steps of 10 degrees intervals at different orientations in magnetic field with an X-band EPR spectrometer. The spectra were found to be slightly anisotropic and temperature independent; taking into consideration the chemical structure with experimental data we found only one free radical structure The radical structure was simulated using the Bruker software WinEPR-Simfonia and the experimental g-value with anisotropic hyperfine coupling constants of radical were calculated as follows: g = 2.0124, aCH(4) = 26.51G, aCH(3) = 5.45G.