(AlAs)1/2(GaAs)1/2fractional‐layer superlattices with a new periodicity perpendicular to the growth direction are successfully grown, by metal–organic chemical vapor deposition on (001) GaAs substrates, slightly misoriented towards [1̄10]. The periodic structure in the lateral [1̄10]direction is analyzed by x‐ray superlattice satellite diffraction and high‐resolution transmission electron microscopy(TEM). Superlattice width along [1̄10] direction are exactly the same as the mean distance of each monolayer step on the (001) vicinal surface. The satellite peak intensity increases with increasing AsH3partial pressure, and is stronger for substrates misoriented towards [1̄10] than those misoriented towards [110]. The result can be explained using the lateral growth model, taking into account the dangling bond at the step edge. The superlattice image is clearly observed by TEM, which shows that the superlattice periods are almost uniform everywhere.