A process of forming an epitaxial substrate is disclosed, where the epitaxial substrate includes a nucleus forming layer made of aluminum nitride (AlN) grown on a substrate made of silicon carbide (SiC). The process includes steps of: (1) first measuring the first reflectivity R0 of a surface of the SiC substrate, (2) growing the nucleus forming layer made of AlN as measuring second reflectivity R1 of a grown surface of the AlN nucleus forming layer, and (3) ending the growth of the AlN nucleus forming layer when a ratio R1/R0 of the reflectivity enters a preset range.