Live screencasting involves encoding and streaming the screen content of a PC in real-time. Most existing H.264 rate control (RC) algorithms are designed for natural scenes and do not perform well with the quite different signal characteristics of screencasting. This paper proposes a content-adaptive H.264 RC scheme which classifies screen content as “slow-motion” phase or “fast-motion” phase on the fly, based on the temporal and spatial frame-area update pattern of recent frames. Then, for frames in “slow-motion” phase, which usually result from a presenter's GUI operations, a new RC algorithm named Frame Rate Adaptive-CQP (FRA-CQP) is applied, which puts priority on the quality of individual frame rather than the frame rate. For frames in “fast-motion” phase, which usually result from playing a movie during the presentation, the classical CRF+VBV RC algorithm is applied. Evaluation results show this adaptive RC scheme can regularly achieves higher subjective quality assessment score (0.7 to 1.6 points on a 1-5 scale) than existing algorithms while meeting the RC objective.