As a major Chinese dialect, Cantonese is well known for its complex tone system. This paper applies the command-response model to represent the F0 contours of Cantonese speech. Analysis-by-Synthesis is conducted on both designed sentences and connected utterances, from which a set of appropriate tone command patterns is derived. By intrinsically incorporating the effects of tone coarticulation, word accentuation and phrase intonation, the model provides a high accuracy of approximation to the F0 contours of Cantonese, and hence serves as a much better tool to quantitatively describe the continuous F0 contours than the traditional tone scale notation system. The constraints in timing and amplitude of tone commands are also investigated, which can be used for rule-based synthesis of F0 contours.