The surface treatment of volcanic ash particles with various concentrations (1-3-5% (v/v)) silane coupling agent (3-aminopropyltriethoxysilane (3-APTS)) having organic functional group was conducted. After surface treatment, polyphenylene sulfide (PPS) composite samples containing surface treated volcanic ash particles at two concentrations (10 and 15 wt%) were manufactured by twin screw extruder and injection molding machine. This study investigated the effect of surface modification on mechanical properties of two different concentrated volcanic ash/PPS composites. All tests were performed using a Shimadzu AG-X Universal Tester. Tensile and three point bending tests were carried out at a crosshead speed of 1 mm/min and 2 mm/min according to ISO 527-2 and ASTM D790 standards, respectively. During the experiments, the relation between the mechanical properties and surface treatment concentrations of silane agent on volcanic ash/PPS composites was determined as a function of tensile strength and modulus, flexural strength and modulus. Moreover, fracture surface morphologies of volcanic ash/PPS composite samples were observed by using a scanning electron microscopy.