Automatic exposure control (AEC) is one of the indispensable functions of modern digital still cameras and video cameras. A new automatic exposure algorithm using luminance histogram of an image is proposed. According to the attention mechanism of human visual system (HVS), it is considered that peak regions in luminance histogram correspond to the regions of no interest in an image. The aim of the algorithm is to find the two largest peak regions in histogram and calculate the mean weighted luminance of entire image. The luminance weights of pixels in peak regions are decided by a set of quadratic curves, and the parameters of quadratic curves are affected by the brightness of image background, which can be obtained from parameters of the automatic exposure system. Fuzzy logic is applied to practical systems to determine the luminance weights of pixels in the two largest peak regions. Results show that the new automatic exposure algorithm gives efficient exposure control over various scene tests.