In this work, radiosensitivies of solid creatine (C-1) and its dehydrated form creatinine (C-2) were studied by electron spin resonance (ESR) spectroscopy after irradiating them at room temperature by gamma rays. Although unirradiated C-1 and C-2 exhibited no ESR signal, irradiated C-1 and C-2 presented ESR spectra of unresolved triplet and doublet appearances, respectively. Variations of the intensities associated with the triplet and doublet spectra with microwave power, temperature, storage time at room temperature, and applied radiation dose were investigated. Three radical species with different decay features were calculated to govern the biphasic decays of the intensities associated with three assigned resonance peaks of C-1. Intensities related with two resonance peaks of unresolved doublet of C-2 were found to stay almost constant at room temperature over a storage period of about 2 months and vary similarly with the increase in microwave power and applied radiation dose. This was evaluated by implying the presence of single type radicals giving rise to the formation of the observed doublet signal of C-2. Radiation yields of C-1 and C-2 were calculated to fall into the range of most materials.