Micromirrors with a tilt angle of 45° are widely used in optical switching and interconnect applications which require 90° out of plane reflection. Silicon wet bulk micromachining based on surfactant added TMAH is usually employed to fabricate 45° slanted walls at the '1 0 0' direction on Si{0 0 1} wafers. These slanted walls are used as 45° micromirrors. However, the appearance of a precise 45° {0 1 1} wall is subject to the accurate identification of the '1 0 0' direction. In this paper, we present a simple technique based on pre-etched patterns for the identification of '1 0 0' directions on the Si{0 0 1} surface. The proposed pre-etched pattern self-aligns itself at the '1 0 0' direction while becoming misaligned at other directions. The '1 0 0' direction is determined by a simple visual inspection of pre-etched patterns and does not need any kind of measurement. To test the accuracy of the proposed method, we fabricated a 32 mm long rectangular opening with its sides aligned along the '1 0 0' direction, which is determined using the proposed technique. Due to the finite etch rate of the {1 1 0} plane, undercutting occurred, which was measured at 12 different locations along the longer edge of the rectangular strip. The mean of these undercutting lengths, measured perpendicular to the mask edge, is found to be 13.41 μm with a sub-micron standard deviation of 0.38 μm. This level of uniform undercutting indicates that our method of identifying the '1 0 0' direction is precise and accurate. The developed method will be extremely useful in fabricating arrays of 45° micromirrors. © 2016 IOP Publishing Ltd.