While the tonal characteristics of Chinese have been qualitatively described in traditional phonetics, a more quantitative analysis requires a mathematical model. We present such a model for the fundamental frequency contours of the Standard Chinese, based on an extension of a model that has already been proved to be applicable to Japanese. This model allows one to interpret a given F0 contour in terms of tone commands and phrase commands, and to analyze various tonal phenomena in quantitative terms. Based on the analysis of a number of words and short sentences using this model, several rules are derived to express various sandhi phenomena found in connected utterances.