Two typical kinds of InGaN quantum dots (QDs) have been grown on sapphires under different conditions through modifying the NH3 flow rate using metal-organic chemical vapor deposition: small spherical dots with a high dot density and large truncated pyramidal dots with a low dot density. The small dots have been found typically coherent and defect-free, while a strain relaxation has often been observed in the large dots. Consequently, this leads to a massive difference in optical properties between them. The optical properties have been investigated by means of temperature-dependent and excitation power-dependent microphotoluminescence measurements. It has been found that the small spherical QDs show higher optical quantum efficiency and much weaker piezoelectric field induced quantum-confined Stark effect than the large truncated QDs. Based on the energy balance between the strain and surface energy, the influence of V/III ratio on the transition from two-dimensional to three-dimensional growth mode during the QD growth has been discussed.