To determine of irradiation effect on 4-hydroxyphenyl-acetic acid polycrystalline, electron paramagnetic resonance (EPR) measurements were carried out. Two samples were used, which were given dose of 22.8 and 49 kGy by gamma rays using ⁶⁰Co-source. EPR signals were not observed from irradiated sample, taken dose of 22.8 kGy. The measurements were performed on the sample, absorbed dose of 49 kGy, at the temperature between 120 K and 450 K. The two radical structures were suggested within experimental error. Though the radicals are identical, it was determined that they have different EPR parameters. It was observed that the intensities of the EPR spectra were to be dependent on the temperature. Also, in this study, it was aimed to test success of the machine learning methods to select the best method can be implemented theoretically.