A composite, a high-frequency circuit substrate using the same and method thereof are discussed. The composite includes the following solid components: a DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide) derivative compound of 10-70 wt %, a curing agent of 10-50 wt %, one or more epoxy of 10-90 wt % and an inorganic filling material of 10-40 wt %. The non-halogen low dielectric epoxy composite uses a high-purity DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide) derivative as tiny particles dispersing in the composite. The crosslinking yield of the composite is not reduced, while the heat resistance and flame retardancy are increased. The prepreg and copper foil covered laminate for use in printed circuit board, made from the epoxy composite, has great dielectric property and high glass transition temperature (GTT), satisfying the need of high frequency in electronic signal transmission and high speed in data processing of the industry of copper covered printed circuit board.