The GaAs(001)-(2 × 4)β 1 surface is analyzed by using ab initio molecular orbital calculation based on a cluster model. The (2 × 4) surface consisting of three surface As dimers and one missing As dimer is modeled both by Ga 13 As 15 H 32 (cluster 1) and Ga 8 As 14 H 20 (cluster 2). For cluster 1, two optimized structures are obtained. One corresponds to a well-known structure which satisfies the electron counting (EC) model. The other is a novel one which does not obey the EC model. The total energy of the novel structure is slightly higher than that of the conventional one, and the conventional structure is shown to be the ground state. For cluster 2, only the conventional structure is obtained. Details of the electronic properties of these surface structures are also discussed.