Optical properties of InGaN quantum dots (QDs) with and without a GaN capping layer have been investigated, showing a major difference between each other. Compared with the InGaN QDs with a GaN capping layer, those grown under identical conditions but without the GaN capping layer showed much stronger photoluminescence (PL) emission and a similar to 350 meV blueshift in emission energy. The excitation power-dependent PL measurements indicated that the emission energy of the QDs with the capping layer showed a large blueshift with increasing excitation power, while there was a negligible shift in the QDs without the capping layer. The major difference between them is attributed to existence of a strong quantum-confined Stark effect (QCSE) in the QDs with the capping layer, while there is no clear QCSE observed in the uncapped QDs. The transition energy has been calculated for both QDs within the framework of effective-mass approximation and variational approach, showing a good agreement with the experimental data. The results obtained should be highly taken into account in investigating the optical properties of InGaN QDs on a GaN surface. (c) 2007 American Institute of Physics.