An improved method of step height measurement by optical frequency comb and tunable diode laser is presented. The tunable diode laser is phase locked to the optical frequency comb, so that the output wavelength of the laser can be locked and measured in a high accuracy. Based on the principle of variable synthetic wavelength chain, the step height can be measured by tuning the wavelength of the locked tunable diode laser. This method can be used to eliminate the effect of the synthetic wavelength error on the measurement uncertainty of the step height. The stability of the phase locked laser is tested by locking a tunable diode laser to an optical frequency comb for about 5000 s. It is shown that the frequency stability of the phase locked laser is 1.810-12. The uncertainty of the step height measurement by this method is estimated to be 7.9 nm accordingly. Moreover, the measurement result of the step height can be traced to the time and frequency standard.