Cesium-based perovskites nanocrystals (NCs) have outstanding photophysical properties (e.g. narrow-band photoluminescence and high quantum yields) improving the performances of lighting devices. Fundamental studies on excitonic properties and hot-carrier dynamics in perovskite NCs further suggest that these materials show higher efficiencies compared to the bulk form of perovskites. However, the relaxation rates and pathways of hot-carriers are still being elucidated. One of the most relevant mechanisms of intraband hot-carrier relaxation is thermalization process coupled with lattice vibrations. By using ultrafast transient spectroscopy and calculating electronic band structures, we investigated the dependence of halide in Cs-based perovskites (CsPbX3 with X = Br, I, or their mixtures) NCs on the hot-carrier relaxation processes. All samples exhibit ultrafast (< 0.6 ps) hot-carrier relaxation dynamics with following order: CsPbBr3 (310 fs) > CsPbBr1.5I1.5 (380 fs) > CsPbI3 NC (580 fs). These result accounts for a reduced light emission efficiency of CsPbI3 NC compared to CsPbBr3 NC.