Under short pulse laser e_xcitat ion, it has been observed, for the first time, a new high-energy photoluminescence emission from GaN_xAs1-_xPGaAs SQWs. This new emission has totally different optical properties compared with the localized e_xciton transition in GaN_xAs1-_x, and is attributed to the recombination of delocalized e_xcitons in QWs. At the same t ime, a competition process between localized and delocalized e_xciton emissions in GaN_xAs1-_xPGaAs quantum wells is observed in the temperaturedependent PL spectra under the short pulse e_xcitation. This competition process for the first time, reveals the physical origin of the temperature-induced S-shaped PL peak shift, which was often reported in the disordered alloy semiconductor system under continuous-wave e_xcitation and puzzled people for a long time. We have also investigated a set of GaN_xAs1-_x samples with small nitrogen composit ion(_x< 1%) by PL, and time-resolved PL.After the PL dependence on temperature and e_xcitation power and PL dynamics were measured, the new PL peak was identified as an intrinsic transition of alloy, rather than N-related bound states. This is the first observation in PL, showing that alloy state e_xists in GaN_xAs1-_x materials even when N composition is smaller than 011%. Finally by select ive e_xcitation, both type-I and type-II transitions were observed simultaneously in GaAs1-_x Sb_xPGaAs SQWs for the first time.