We report on the quantitative study of microtwins (MT) defects in the GaP/Si(0 0 1) thin film grown by Molecular Beam Epitaxy and the optical properties of GaAsP(N)/GaP(N) quantum wells grown on top of the GaP/Si pseudo-substrates. A 780 nm photoluminescence at room temperature from the GaAsPN quantum wells is measured on silicon. Time-resolved photoluminescence has been performed and evidences the influence of non-radiative defects originated from the GaP/Si interface. The structural defects such as MT are quantitatively analyzed by synchrotron X-ray diffraction (XRD) combined with transmission electron microscopy (TEM) analyses. We show that the XRD measurements are in good agreement with TEM observation and reveal a strong contribution of MT in the [1 1 1] direction. The MT density appears to be directly correlated with the growth temperature.