The effect of carrier diffusion on threshold current is studied quantitatively for the oxide-aperturbed 780 nm vertical-cavity surface-emitting lasers (VCSEL) from the measured spontaneous emission. The oxide aperture size of 2–10 &mgr;m is prepared and characterized. The diffusion coefficient of4.6 cm2/sand the nonradiative recombination coefficient of6.8×107/sare obtained using the independence of the nonradiative recombination coefficient on the laser aperture size. The contribution of carrier diffusion loss is 42&percent; of the threshold current for the 2 &mgr;m VCSEL. The relative contribution of the diffusion becomes smaller for the larger devices as is expected. ©1997 American Institute of Physics.