The effect of the fly ash (FA) filler modified by sulphuric acid on the mechanical and physical properties of poly(ethylene terephthalate) composite material (PCM) has been investigated. The results of the presented study can be formulated as follow. The element and oxide composition of original (as received) fly ash is affected by the modification of sulphuric acid of 5 % concentration. At the same time, the variations in the particle size distribution of filler can be observed. In particular, the modified FA filler has an increased content of fine particles. The XRD pattern of as received fly ash indicates the presence of both crystalline (25 %) and amorphous phases (75 %) while XRD pattern of FA after chemical modification points to the fact that the content of crystalline phase has been increased up to 40 % but the content of amorphous phase has been decreased up to 60 %. The results of mechanical and physical properties of PCM with various content of unmodified fly ash filler (55, 60, 65, 70, and 75 %) indicate that compressive and flexural strength tends to rise with increasing concentration from 55 to 65 percent. In the certain case when the fly ash filler is modified by sulphuric acid of 5 % concentration, the values of compressive and flexural strength of PCM at 65 % FA loading is higher than compared with PCM on the base of unmodified filler. The strength gain may be attributed to the increasing crystallinity in the recycled poly(ethylene terephthalate) (rPET) matrix of the composite material as the fly ash content is increased. The XRD, DTA/TGA data of pure rPET and PCM are in conformity with the results of mechanical and physical properties of PCM.