Manual adjustment of lighting intensity to achieve high-quality images brought about functional limitations in fully automatic micro-assembly and micro-manipulation.An illumination optimum method based on image analysis in spatial domain or frequency domain was presented.An illumination auto adjustment system was designed and realized to test and solve above issues.Linearity of image average grayscale under the adjustment can reach 1.3%.This paper firstly proposed image quality assessment operators based on high frequency of microscopic images.Illumination intensity can be adjusted automatically by ways of image analysis.Illumination evaluation functions A_{con_Tam} and A_std were presented.Experimental results show that the correct illumination configurations are especially important for high resolution microscope and the quality of illumination directly affects the accuracy and repeatability of micromanipulation.