In this paper, we present a novel Intra-rate estimation method for H.264 rate control. The proposed method establishes the Intra-frame Rate-Quantization (R-Q) estimation model by using gradient-based picture complexity measure. The proposed model aims at selecting accurate quantization parameters for Intra-coded frames which can fully comply with buffer constraints. By adaptively updating this R-Q model, the performance of the model can be guaranteed regardless of the changing of video frame characteristics. Simulation results show that by using our proposed scheme, better rate control for Intra-frames and frames at scene changes can be achieved. As a result, the number of skipped frames is significantly reduced thus it provides improved visual quality of the reconstructed pictures.