Dynamical behavior of optical gain formation has been assessed at room temperature in the InxGa1-xN multi-quantum-well (MQW) based laser diodes (LDs) by employing pump and probe spectroscopy with a pulse width of 150 fs. The LDs are composed of (a) In0.1Ga0.9N–In0.02Ga0.98N MQW and (b) In0.3Ga0.7N–In0.05Ga0.95N MQW, whose stimulated emissions correspond to near ultraviolet (390 nm) and blue (440 nm), respectively. The optical gain was contributed from the nearly delocalized states [the lowest-quantized MQW levels (LQL)] in the sample (a), while it was from highly localized levels with respect to the LQL by 500 meV for the sample (b). It was found that the photogenerated carriers rapidly (less than 1 ps) transferred to the LQL, and then relaxed to the localized tail within the time scale of about 5 ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photoexcitation. © 2000 American Institute of Physics.