Three CdTe quantum dots and CdTe/CdS quantum dots with different sizes were synthesized by hydrothermal method. The nonlinear absorption properties of CdTe quantum dots with three different sizes were investigated by the Top-hat Z-scan technique under ns, ps and fs laser pulses. The experimental results show that the nonlinear absorption properties of CdTe quantum dots with three different sizes are saturated absorption under different laser pulses, and all of them show the increasing tendency of nonlinear absorption characteristics with the decrease of quantum dots size. In order to further study the effect of quantum dots size on the nonlinear absorption properties, the nonlinear absorption properties of CdTe/CdS quantum dots were investigated under fs laser pulse. With the increase of cladding time, the thickness of shell increases, the size of quantum dots increases, and the nonlinear absorption properties decrease. And the nonlinear absorption properties of CdTe/CdS quantum dots were better than those of CdTe quantum dots. The nonlinear absorption properties and the quantum size effect mechanism of CdTe quantum dots and CdTe/CdS quantum dots were discussed, the experimental results show that the synthetic quantum dots have good quantum size effect.