The third-order nonlinear optical susceptibility X(3), nonlinear refractive index n2 and time response property of 1,2,5,6,dibenzathracene (C22H14) and dibenzopentacene (C30H18) in fused-cyclic aromatic hydrocarbon were studied with forward degenerate four-wave mixing (DFWM). The mechanism for their larger nonlinear effect and faster time response was discussed.