Dosimetric properties of gamma irradiated Isoniazid (IS) were investigated through Electron Paramagnetic Resonance (EPR) spectroscopy. While unirradiated IS presented no EPR signal, irradiated IS exhibited a significant EPR spectrum at room temperature. Some EPR parameters of obtained experimental spectrum were calculated. In order to describe the variation of EPR signal intensity with absorbed radiaiton dose, several mathematical equations were tried. It has been determined that the best fit of IS sample is with polinomial function (R2 = 0.9675). Moreover time dependency of the intensity of the EPR signal of the irradiated sample was analysed to study the stability of the free radicals.