This paper reports the results of X-ray multi-crystal diffractometry analysis of heavily Te-doped (1 0 0)-GaAs epitaxial thin layers grown by intermittent injection of TEGa/AsH3 in an ultra high vacuum. It is shown that the differential lattice strain in < 100 > direction was increased monotonically with increase of impurity concentration in the range of 5 x 10(19)-5 x 10(20) cm(-3), but full-width at half-maximum of rocking curve was kept constant at about 360 s of are. Independent of the impurity concentration, reflection X-ray topography does not reveal the formation of misfit dislocation. Reciprocal lattice mapping method is also applied to clarify the origin of double peaks in X-ray rocking curve. In addition with electrical measurement results, defect formation mechanism in heavily Te-doped GaAs thin layers are discussed.