Although many histogram equalization (HE) based methods have been developed to overcome the problem of mean-shift, they often suffer from over-enhancement caused by an abrupt jump of gray levels. Consequently, mean brightness cannot be preserved with higher accuracy. In this paper, one simple and efficient method, brightness-preserving weighted subimages (BPWSI), for brightness preservation and contrast enhancement of gray level images is proposed. First, based on brightness preserving bi-histogram equalization, two new subimages are defined. Then one brightness-preserving way to calculate two weight coefficients is given. Finally, the output image is defined as the weighted sum of two defined subimages. Extensive simulations show that BPWSI can preserve the mean brightness with higher accuracy than many other HE-based methods while enhancing the contrast efficiently. Furthermore, due to its simplicity, BPWSI can be implemented on real-time systems.