Up-converted photoluminescence (UPL) was observed at the long-range ordered (Al0.5Ga0.5)0.5In0.5P/GaAs(001) heterointerface, during the excitation of GaAs. Excitation-power dependence of the UPL intensity reflects carrier-localization properties caused by potential fluctuations due to a multidomain structure in the ordered (Al0.5Ga0.5)0.5In0.5P. When we excited the GaAs layer, photoexcited carriers spatially transferred to the (Al0.5Ga0.5)0.5In0.5P layer and relaxed from higher lying states to lower lying states in the fluctuated potential. Time-resolved measurements were performed for the UPL and normal photoluminescence (NPL) excited by an above-gap light. We observed a slowly rising component in the time-resolved UPL, whereas the NPL showed an exponential decay profile. These results reveal that the carrier-relaxation processes are different near the surface and near the interface of the epitaxial layer.