An optical fiber dual-wavelength interferometric sys tem,which is suitable for on- line measurement of step heights with high precision and without the need of eac h wavelength working individually in sequence to perform the measurement,is proposed. By employing the characteristic of a fiber Bragg grating of reflecting Bragg wave length,two different wavelengths are working simultaneously and independently in the same optical fiber interferometric system,and two interfero metric signals of each wavelength can be detected simultaneously and respectively.The measurement range is expa nded to half of a synthetic wavelength. The system is stabilized by a feedback loop which compensates the influence ind uced by the environmental disturbances,and thus the system is robust enough for on -line measurement.The system is capable of measuring st ep heights as large as 1mm and the measurement resolution is less than 1nm.