A surface fractal model was presented to describe the interface in block copolymers. It gives a simple power-law relationship between the scattering intensity I(q) and the wave vector q in a relatively wide range as qxi >> 1, I(q) is-proportional-to q(D-6), where xi is the length characterizing the size of the structure producing scattering, D is the fractal dimension of the interface. Small-angle X-ray scattering experiment results of both linear and star styrene-butadiene-styrene triblock copolymers show that the model is quite reasonable. It seems that the interface constructed by the joints of different blocks is a fractal surface with a fractal dimension D from 2 to 3.