[en] The optical properties of InGaAs/GaAs quantum dots (QDs) were investigated by temperature dependent photoluminescence (PL) and photoreflectance (PR) spectroscopies. The surface morphology and structure analysis of InGaAs QDs were also examined and characterized by a field emission scanning electron microscope (SEM) and an atomic force microscope (AFM). The In_0.5Ga_0.5As/GaAs self-assembled QDs specimens were grown with gas-source molecular beam epitaxy and migration enhanced techniques. The area density of the QDs is on an order of magnitude about 1 x 10¹⁰ dots/cm². The measured PL results exhibited 5 major energy peaks, two of which are attributed to InGaAs QDs, one is attributed to the InGaAs wetting layer and the other two are attributed to GaAs band-gap transitions. Two of the low energy features are identified to the optical transitions of the ground state. They were originated from the two kinds of InGaAs QDs which might be formed with slight change of the indium composition. An inverted ' S curve' shape of the temperature dependent PL peak energies was observed. This abnormal behavior of the line-shape is attributed to carrier localization. The results of PR measurement which reveal energy features on the high energy side contributed by GaAs is also reported