On the basis of measurement and theoretical analysis, the linear and nonlinear optical properties of the CdSe quantumn dots with different nanoparticle sizes were investigated. The relation between the nanoparticle size and the absorption peak of CdSe quantumn dots was characterized by UV-VIS-NIR spectroscopy, and the dependence of third-order nonlinear susceptibility of quantumn dots on nanoparticle size was investigated by picosecond Z-scan technique with different excitation wavelengths (532nm and 1064nm,resp.). The analysis of optical properties of the films was carried out using the theory of localized field enhancement. The experimental and theoretical results showed the dependence of third-order nonlinear susceptibilities χ(3) on the CdSe nanoparticle size. With CdSe nanoparticle size around 4.3nm and the excitation wavelength of 532nm, which was close to the resonance, the third-order nonlinear susceptibility χ(3) showed a maximum value of 2.0×10-11 esu.