Optical investigation under high excitation is carried out on InGaN/GaN multiple-quantum wells (MQWs) with different well thickness. In the case of MQWs with wide well thickness, a strong emission from quantum-dot-like states is observed, appearing only under high excitation on the high energy side of the emission from the transition between the first electron and first heavy-hole subbands (e1-h1) as confirmed by detailed calculation. The emission from the quantum-dot-like states is proved by temperature- and power-dependent photoluminescence measurements. The e1-h1 transition dominates the emission under low excitation, whereas in contrast the emission under high excitation is dominated by the quantum-dot-like states from low temperature to room temperature, which can partly explain the dramatically enhanced internal quantum efficiency at room temperature under high excitation, compared to the case under low excitation. Since the lasing process in InGaN/GaN-based MQWs generally takes place under high excitation, the result should be highly taken into account for understanding the emission mechanism during lasing process. (C) 2004 American Institute of Physics.