Cu3Ga5S9 semiconducting single crystals were investigated using thermoluminescence (TL) measurements in 10-300 K temperature region. In the TL glow curve, one peak starting to appear at the instant temperature is increased from 10 K and another peak, which is broader than a general individual TL peak, were observed. The broad peak around 66 K was investigated using T-m-T-stop experimental method to understand whether or not this peak is composed of more than one individual peaks or continuously distributed traps. Curve fitting, initial rise and peak shape methods were used for acceptable TL curves to be analyzed. TL curves in T-m-T-stop method indicated that observed peaks are due to the existence of quasi-continuous distribution of traps. Structural characterizations of Cu3Ga5S9 single crystals were studied using x-ray diffraction and energy dispersive spectroscopy measurements. The crystal structure, lattice parameters and atomic composition of the elements were reported in the present paper.