By precise measurement of the spatial variation in the lattice parameters of {001} undoped Czochralski grown gallium arsenide wafers, it was found that the lattice parameters are strongly dependent upon elastic strain and not so much on the melt composition. Although a remarkable spatial variation in the lattice parameters of ±5×10−6nm from the average value was observed in a wafer specimen, the variation was considerably reduced to ±2×10−6nm by dividing the wafer into small specimens. The lattice parameter variation was smaller than 2×10−6nm due to a change in the As composition ratio from 0.42 to 0.52 in the melt.