Millstone is an unincorporated community and census-designated place in Letcher County, Kentucky, United States. Its population was 117 as of the 2010 census. Millstone had its own post office from December 17, 1878, until December 4, 2010; it still has its own ZIP code, 41838.According to the U.S. Census Bureau, the community has an area of 0.222 square miles; 0.214 square miles of its area is land, and 0.008 square miles is water.