Thermal properties of volcanic ash filled polyphenylene sulfide (PPS) composites have been investigated with respect to surface treatment that was conducted with 3-aminopropyltriethoxysilane (3-APTS) which had an organic functional group. Volcanic ash/PPS composite samples were prepared by using DSM Xplore 15 ml twin screw microcompounder and DSM Xplore 12 ml injection molding machines. The content of volcanic ash in composite samples was varied as 10 and 15 wt%. Volcanic ash filler dispersion and adhesion between volcanic ash particles and PPS matrix were examined by scanning electron microscopy. Thermal properties such as crystallization and melting behavior were investigated by TA Instruments Q200 differential scanning calorimeter. According to the test results, the relation between the thermal properties and surface treatment was determined as a function of melting temperature and melting enthalpy. Additionally, crystallization behavior was investigated according to surface treatment.