Abstract: This paper presents He-Ne laser nanometer scale measurement system based on orthogonal polarization dual longitudinal mode. With inserted birefringence element, frequency splitting effect will occur inside He-Ne resonant cavity, turning the laser into dual-frequency laser with adjustable frequency difference. By using effect of laser physics such as frequency splitting effect, mode competition effect and double longitudinal mode power tuning, and setting floating threshold, a new type of laser nanometer measurement ruler is proposed. The laser wavelength is applied as a ruler with traceability, leading to nanometer scale resolution without electrical subdivision. Experimental results of comparing with laser interferometer show that system optical resolution is 79 nm, measuring range is 15 mm, linearity is 5.4×10^-5, and standard deviation is 380 nm.