We present an efficient rate control strategy for H.264 in order to maximize the video quality by appropriately determining the quantization parameter (QP) for each macroblock. To break the chicken-and-egg dilemma resulting from QP-dependent rate-distortion optimization (RDO) in H.264, a preanalysis phase is conducted to gain the necessary source information, and then the coarse QP is decided for rate-distortion (RD) estimation. After motion estimation, we further refine the QP of each mode using the obtained actual standard deviation of motion-compensated residues. In the encoding process, RDO only performs once for each macroblock, thus one-pass, while QP determination is conducted twice. Therefore, the increase of computational complexity is small compared to that of the JM 9.3 software. Experimental results indicate that our rate control scheme with two-step QP determination but single-pass encoding not only effectively improves the average PSNR but also controls the target bit rates well.