Abstract: A novel bi-histogram equalization algorithm is proposed for keeping image brightness preservation. Based on the grayscale histogram of the given image, the corresponding relation between the grayscale threshold and the output brightness is established; all the thresholds are calculated to meet the requirements of the output brightness, then the best one is chosen for bi-histogram equalization so that the biggest histogram similarity exists between the output image and the result by the brightness preserving bi-histogram equalization (BBHE) method. Experimental results of several images with different brightness show that by using the proposed algorithm the margin of relative error between output brightness and expected value reaches 0.43%~2.56%, but only-36.94%~8.85% by using the BBHE method. We conclude that the proposed algorithm significantly improves the accuracy of brightness control target for bi-histogram equalization, and the operating speed meets real-time requirements.