Temperature dependence of threshold current in InGaAsP/InP double‐heterostructure (DH) lasers was studied from the standpoint of the effect of carrier leakage from the quaternary active region into the InP confining layers. The carrier‐confinement coefficient, defined as the ratio of confined carriers to total injected carriers in the active region, was connected with other oscillation characteristics such as emission efficiency, carrier lifetime, and internal quantum efficiency in three different ways. These variations, as a function of ambient temperature, were measured and the temperature variation of the carrier‐confinement coefficient was evaluated and compared with that of threshold current. These results demonstrated that the carrier leakage was the dominant mechanism on temperature dependence of threshold current in InGaAsP/InP DH lasers. Moreover, we too discussed the other possibilities such as the effects of interfacial recombination at heterojunctions and laser parameters.