Single crystals of non-linear optical (NLO) material 3-methoxy-4-hydroxy benzaldehyde (MHBA) were grown following slow evaporation method. The grown crystals were characterised by the measurement of unit cell dimensions single crystal X-ray diffraction, density, melting point and X-ray powder diffraction pattern. Hardness study for the grown crystals was carried out. The grown crystals were gamma -irradiated to produce free radicals and were analysed by electron paramagnetic resonance (EPR) technique.