Chemically cleaving-etch method was presented to form relief modulation in gelatin based on the analysis on the cleavage of protein. Thank to the much fewer number of cleavage site for chemical cleavage than that for enzyme, chemical cleavage method was of higher etching resolution and better profile fidelity. The predication was proved to be right and experiments showed that relief modulation resolution was got to 500 lines/mm spatial frequency, which was much higher than enzyme method. With the projection exposure system, a micro-prism array with 30 μm wide and 0.8 μm high cell dimension was fabricated using the chemically cleaving-etch method.