An under-floor frame system is disclosed. In particular, a main frame made of carbon fiber composite materials forms a closed quadrangle. A center cross member made of carbon fiber composite materials runs along a width direction of a vehicle and connects middles of the both side members. Also, a front cross member and a rear cross member made of carbon fiber composite materials runs along a width direction of a vehicle and connects a front side member portion and a rear side member portion at both sides of the main frame, respectively. A center tunnel member made of carbon fiber composite materials and is extended in a width direction of a vehicle to traverse an upper space of the center cross member and connects the front cross member and the rear cross member.