The evolution of GaAs ridge structure formation by molecular beam epitaxy on a patterned substrate has been investigated using an ultrahigh vacuum atomic force microscope. It is found that the morphology of ridges can be quite irregular with random formation of various facets in the intermediate phase of growth, but self-smoothing processes of the lateral facets take place later on, leading to very sharp and smooth ridge structures in the end. The ridge top is quite sharp and straight with the height fluctuation of within 1–2 nm over the length of 1.4 &mgr;m. The role of the Ga atom flows from the side (111)B surfaces to the top (001) surface and their local modulations are considered to account for these observations. ©1997 American Institute of Physics.