A one-pass picture-layer rate control (RC) method and system for controlling a video encoder: The bit allocation and determination of quantization parameter (QP) in this rate control method and system are based on a Rate-Quantization (R-Q) scale model. The R-Q model is a linear one, wherein the number of bits per picture is in reverse ratio of quantization scale for that picture. A conversion of exponential-based QP to linear-scaled QP is taken in the calculation of complexity model and determination of QP. The conversion is integer-based and is consistent with the quantization array defined in the H.264 standard.