This paper presents a fast rate control algorithm in a unit-layer for H.264/AVC video coding. The statistical relation between the quantization parameter (QP) and the encoded bits of a unit is derived for inter-coded frames. In addition, a new complexity estimation is defined to determine the weights of complexity of frame layer and unit layer. Using the statistical properties and complexity updating process, the number of encoded bits was determined as a function of the QP, leading to a significant computational cost reduction, stable buffer management, and automatic detection of region of interest (ROI) without extra computational cost. Experimental results show the capability of the proposed algorithm.