Transport properties of electrons in a novel in‐plane superlattice structure are studied and the field‐effect transistor action has been demonstrated. The structure consists of ann‐AlGaAs/GaAs modulation‐doped heterojunction in which an array of monolayer‐thick AlAs grid is embedded with an average period of 162 A˚ in the channel region of the heterojunction. This grid has been prepared with molecular beam epitaxy by depositing a half monolayer of AlAs on a GaAs (001) vicinal plane, where periodically spaced atomic terraces are formed. The electron mobilities &mgr;∥,&mgr;⊥parallel and normal to the grid are measured as functions of electron concentrationNS. While the mobility ratio (&mgr;∥/&mgr;⊥) is nearly unity at lowNS, the ratio is found to get as large as 2.2 asNSincreases. This anisotropic behavior of &mgr; is well accounted for by the calculated nonparabolicity in the miniband structure of in‐plane superlattices.