The tensor LEED analysis of the intensities of electron beams diffracted from the GaAs(0 0 1)-c(4 × 4) grown by molecular beam epitaxy (MBE) has been performed. Surface structures with symmetrical and asymmetrical 3-dimer models in the topmost layer have been investigated. The best-fit structure with central dimer compressed with respect to the As4 molecule by 20% has been found. Model with asymmetrically arranged dimers fits experimental data better than that with symmetrical alignment.