A thermochemical heat transfer model in oxygen-assisted laser cutting of carbon steel has been developed in terms of the laser mode pattern, the power density, combustion reaction, kerf width and cutting speed. This model emphasizes the chemical combustion effect as well as the laser mode pattern, which are usually neglected by most existing laser cutting models. Good agreement was obtained between theoretical and experimental results, indicating that approximately 55–70% of the cutting energy is supplied by the combustion reaction of the steel with oxygen, which is consistent with experimental data obtained by other investigators.