Helium atom scattering and low-energy electron diffraction have been used for studying the adsorption of TCNQ molecules on Cu(100). By optimizing the TCNQ growth procedure through the analysis of He-diffraction spectra, a long range (over 2000 Å), ordered molecular overlayer has been attained by TCNQ deposition at 110 K followed by thermal annealing at 315 K. By contrast, no He-diffraction spectra could be obtained by employing growth methods reported previously. Moreover, TCNQ molecules were found to form a single-domain incommensurate overlayer on Cu(100). The lattice parameters of this structure have been determined from an analysis of the measured high-resolution He-diffraction spectra.