The single crystals of wide- and indirect-bandgap semiconductor CdI2 were grown and their optical properties as well as the defect-induced excitonic photoluminescence (PL) spectra were studied. The multiphoton excited PL spectra, charactering the emissions from excitons in the visible region, were taken. The PL intensity (IPL) was found to vary nonlinearly with pumping power (P). The IPL was also found to decrease with increasing temperature. The temperature dependence of IPL for each P followed a fashionable relationship, from which the activation energy (∆E) of the defect-induced excitonic trapping was calculated. The ∆E was found to slightly decrease with increasing P, with an average value of 2.5 meV. The results, however, demonstrates the existence of the self-trapped excitons responsible for the characteristics broad-band emission in the visible range. This study shows that the single crystals of CdI2 might have potential in applications as thermo-luminescent dosimeters in radiation measurements.