The temperature range for distributed feedback (DFB) mode oscillations in 1.5-µm InGaAsP/InP DFB lasers was investigated. The range was determined by the relation between the net gain spectra and the loss difference between the Fabry-Perot and DFB modes. The temperature dependence of the net-gain spectra of 1.5-µm InGaAsP/InP Fabry-Perot (FP) lasers were measured. The loss difference between FP and DFB modes was calculated while taking into account the corrugation phase at facets, the facet reflectivity, and the coupling coefficient. From these results, it is confirmed that DFB lasers with two cleaved facets and a kL value of 2(k: Coupling coefficient, L: cavity length) can oscillate in the DFB mode in a temperature range of at least 100 degrees.