Using low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) it has been found thatglycine molecules adsorbed on Cu(001) can form but only the (2×4) and c(2×4) superstructures. On the basis of themissing LEED spots of the surface, it has been concluded that: each (2 ×4) unit cell consists of two molecules, one beingthe mirror image of the other; the C-C axis of both molecules lies in the mirror plane of the Cu substrate without asignificant shift and twist from the plane; and the two O atoms of the carboxylate group of both molecules locate at thesame height level without significant buckling. According to these conclusions, a structural model has been proposed forthe (2×4) superstructure (a model for the c(2×4) superstructure already exists). We argue that the (2×4) and c(2×4)superstructures must have similar specific surface free energy, that their hydrogen bonds must be of N-H-OⅡ type, andthat their local adsorption geometry must be similar or even the same. The advantage of combining STM with LEEDto determine surface structures is clearly demonstrated.