Three-dimensional etch rate distribution of Si is necessary in analyzing the anisotropic etching of Si. As three-dimensional etch rate distribution can be composed by a series of two-dimensional distributions, a novel method is suggested in this paper for measuring the two-dimensional distributions with simple tools and high efficiency. For example, vertical sidewalls of a series of U-shaped trenches in (0mn) silicon wafers are first created by deep reactive ion etching (DRIE) technique. By measuring the etch rates in normal directions of the vertical sidewalls, two-dimensional distributions of etch rate in (0mn) planes can be found. As the height of vertical sidewalls formed by DRIE can be very large, the planes of sidewall can withstand a relatively long etch time before they are replaced by emerging slow etching planes. No special measuring facilities but a conventional microscope is needed for the measurement. Presented in this paper are etch rate distributions in (001) and (011) crystal planes in 40% KOH and 25% TMAH.