In this paper, we propose an absolute length measurement method with sub-nanometer accuracy by combining a regular crystalline lattice with a scanning tunneling microscope (STM) and the phase modulation homodyne laser interferometry. An arbitrary length can be determined using fine scale (=lattice spacing on crystalline surface) and coarse scale (=wavelength). With the method, an arbitrary length can be represented as Nλ+Md, where N and M are integer, λ is the wavelength and d is the lattice spacing. The phase modulation homodyne laser interferometry can determine the length of interest with picometer resolution, when the optical path difference is the wavelength times integer. The length shorter than the wavelength can be determined using the regular crystalline surface and STM. For the preliminary experiment, the measurement instrument for lattice spacing on crystalline surface was developed and evaluated.