First-principles pseudopotential calculations were carried out to investigate atomic and electronic structures of small GaAs clusters containing even numbers of atoms. The most stable atomic structure of the GaAs cluster significantly depends on its size and composition. The structure becomes substantially distorted as the cluster size increases. The binding energy per atom increases as the size of cluster increases. As-rich clusters have larger binding energies than Ga-rich ones. A distribution of small GaAs clusters obtained in laser vaporization experiments is well explained via inequalities between binding energies. Ionization energies and electron affinities of the stoichiometric GanAsn (n = 2-6) clusters were evaluated.