In this article we present the first observation of high-energy positron diffraction from a clean Si(I I 1)7x7 surface. A 20 keV positron beam with a sufficient coherence length was reflected at the surface and a diffraction pattern was observed. The fractional order Laue zones, which were assigned to be the 1/7-th, 2/7-th and 3/7-th orders from the kinematical consideration were well seen. The rocking curve (diffraction intensity versus glancing angle plot) was also measured. In the total reflection region of the rocking curve, the intensity distribution was hardly reproduced when adopting the distance between the adatom. and stacking fault layers and also the damping potential due to the electronic excitation determined previously. Possible reasons are discussed.