Inorganic perovskites such as CsPbBr have attracted intensive research attention due to their excellent thermal stability and high photoluminescence quantum efficiency. With the decrease of the material's dimension, quantum confinement effects will take place, which will induce many interesting properties. Here, quasi two-dimensional perovskite nanoplatelets (NPLs) and three-dimensional perovskite nanocrystals (NCs) have been grown, and their optical properties have been compared and discussed. It is interesting to find that the photon energy of CsPbBr NPLs hardly changes with temperature due to strong electron-phonon interaction. At the same time, the strong quantum confinement effect results in a smaller Stokes shift and a faster fluorescence lifetime. In addition, although the NPLs have many surface defects, they exhibit a relatively strong negative thermal quenching effect of emission. These results demonstrate the unique optical properties of perovskite NPLs that can pave the way for future nanoscale devices.