The nonlinear optical absorption of CdSe/ZnS quantum dots in toluene with two different sizes are investigated using open aperture Z-scan with 6ns laser pulse at 532 nm. Nonlinear optical absorption of two samples display a switch over from saturation absorption (SA) to reverse saturation absorption (RSA) with increasing input laser intensity. When laser wavelength (532 nm) is smaller than the absorption peak (553 nm), it behaves two different SA processes which are related to different excited states at low intensity and RSA caused by excited state absorption (ESA) at high intensity. When laser wavelength is larger than the absorption peak (503 nm), it behaves SA related to one excited states at lower intensity and RSA caused by excited state absorption (ESA) and two photon absorption (TPA) at high intensity. Our results show that quantum dots are good candidate materials for the fabrication of optical switching and optical limiting device.