The critical importance of the relative position of the reflectors and grating in InP/InGaAsP DFB laser diodes have been directly verified. The relative position was varied by etching the cleaved facet of a DFB laser with the precisely controlled ion beam etching technique. The threshold current, oscillation wavelength, stopband width, and spectral intensity ratio of both modes which form a stopband were measured. All these characteristics changed periodically, with the period being about 2400 A. This value corresponds to one half of the oscillation wavelength in the laser cavity. These characteristic variations resulting from the relative position change of the reflector and grating are theoretically analyzed by the eigenvalue equation which determines the propagation modes. The calculated results qualitatively agree with the experimental findings.