The two-photon absorption (TPA) of colloidal CdS semiconductor microerystai-lites has been studied by using a single-beam transmission technique. For different crystallite sizes of 5.5～11.0nm, the TPA coefficients of 1.31 to 5.24cm/GW are measured and the corresponding x(3) of 2.8×10-12 to 1.1×10-11 esu are obtained at wavelength of 510.6nm. The dependence of crystallite size on TPA has been analysed, and the theoretical prediction is in agreement with the experimental result.