H.264 is the latest international video coding standard. It is a joint work by the ITUT Video Coding Experts Group and the ISO/IEC Moving Picture Experts Group. H.264 contains a number of new features which make it compress video sequences much more effectively than previous standards and can provide more flexibility for application to a wide variety of network environments. But the H.264 standard also brings much more computations, especially when the rate-distortion optimization function is on. In order to reduce the complexity of rate-distortion cost of the rate part, we propose al rate estimation methods to avoid doing the real entropy coding method. The estimation method proposed is based on the notification of CAVLC tables, and is changed adoptively according to total_coeff. The final result shows that the proposed estimation method can reduce the encoding computation by 50% through the intra coding with ignorable degradation of coding performance.