The photoluminescence (PL) spectrum from ensembles of InAs/GaAs quantum dots (QDs) is calculated. The effect of the dot size distribution and the variation of the associated confining potentials on the PL spectra are estimated. It is found that the intermixing of the interfaces causes an increase of the PL spectra energy. The size distribution determines the spectrum width and makes the PL line shape asymmetric with a high-energy tail. Moreover, the non-uniform size distribution also results in a redshift of the PL peak. The experimental PL spectrum is well explained by the size distribution and intermixing effect within the effective mass approximation.