Block copolymer lithography is a promising method for fabricating periodical nano patterns less than 20nm by self-assembly and can be applicable for fabricating patterned magnetic media with recording density over 1Tbit/inch2. We found a simple technique to control the orientation of cylindrical microdomains in thin films. Only by mixing polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers with the constituent homopolymer (PS or PMMA), we could align the cylindrical microdomains perpendicular to the film surface. The added homopolymer induces conformational entropic relaxation of the block chains in microdomain space and stabilizes perpendicular orientation of hexagonally packed cylindrical microdomains. Thus formed perpendicular cylinders can be readily aligned in a regular array with a grating substrate.