We present an oblique microstructure, a tetrahedral three-facet micro mirror, and its fabrication process with the inclined deep X-ray lithography (DXRL). The mirror has an equilateral triangular base of 100 mum length and mirror-like three side-facets inclined to the base at 45 degrees with knife edges. When two equilateral triangle patterns of gold absorber in the X-ray mask shade the PMMA (polymethylmethacrylate) substrate with two times of inclined DXRL at angles of 45 degrees and tan(-1)(2)degrees, the shaded part of PMMA produces a tetrahedral three-facet mirror. The process has been conducted with "a" X-ray mask without any alignments that may degrade the precision of microstructures. The completed tetrahedral mirror shows excellent aspects of mirror-like facets and knife-edges with a surface roughness of 20-30 nm. By changing the gap distance between the X-ray mask and the PMMA substrate, the size of the three-facet mirror can be controlled. The three-facet micro mirror is devised as a three-directional laser beam reflector for the measurement of 6 d.f. motion of the slider flying on a magnetic disk of a hard disk driver (HDD). (C) 2001 Elsevier Science B.V. All rights reserved.