This paper aims at improving perceptual quality of encoded video sequences, and proposes a new perceptual bit allocation scheme for H.264. Firstly, a new motion complexity measure is defined to represent the amount of motion contents between two consecutive frames, and is used to estimate the target bit at frame level. Secondly, a segmentation method exploiting the perceptual characteristics of the video contents is presented, and is used to adaptively update the Lagrangian multiplier in the coding mode selection at macroblock level. Thirdly, based on the motion complexity and Lagrangian multiplier updating, a rate control scheme for H.264 is proposed. Experimental results show that our scheme can effectively improve the perceptual video quality as compared with the H.264 adopted rate control algorithm. Moreover, our scheme also achieves an average peak signal-to-noise ratio gain of up to 0.138 dB for the test sequences.