An epitaxial structure includes a substrate, a nucleation layer, a buffer layer, and a nitride layer orderly. The nucleation layer consists of regions in a thickness direction, wherein a chemical composition of the regions is Al(1−x)InxN, where 0≤x≤1. The x value consists of four sections of variation along the thickness direction, in which a first fixed region has a maximum value, a first gradient region gradually changes from the maximum value to a minimum value, a second fixed region has the minimum value, and a second gradient region gradually changes from the minimum value to the maximum value. An absolute value of a gradient slope of the first and second gradient regions is 0.1%/nm to 50%/nm. A surface roughness of the nucleation layer in contact with the buffer layer is greater than that of the buffer layer in contact with the nitride layer.