A laser-frequency-based displacement measurement system with sub-nanometer uncertainty using an optical frequency comb generator is developed. In this method, the optical frequency of a tunable laser is locked to the resonance of a Fabry-Perot cavity. One of the two mirrors of this Fabry-Perot cavity is connected to the element whose displacement is to be measured. Wide range optical frequency and displacement measurements were realized by using an optical frequency comb generator, which consists of an electro-optic modulator placed inside of an optical resonator. We demonstrate a displacement measurement of up to 10 mum with 220 pm uncertainty under the stable condition.