The theoretical study on the threshold current density (J(th)) of InxGa1-xAs/InGaAsP quantum well (QW) lasers lattice matched to GaAs is presented. The results are also compared with those of InxGa1-xAs/GaAs QW lasers. For relatively long cavity length (>1000 mum), two lasers have similar values of the J(th) for x<0.25, in particular, for the wavelength of 0.98 mum. On the other hand, for x>0.25, InGaAs/InGaAsP lasers show lower values of the J(th) than InGaAs/GaAs lasers for all cavity lengths. The results indicate that InGaAs/InGaAsP lasers are expected as a promising light source for optoelectronic applications.