摘要： In both the nuclear and offshore sectors, there are many potential applications for underwater cutting. Although underwater cutting using lasers was demonstrated many years ago, the limitations imposed at that time by the 10 micron laser beam wavelength available, meant realisation of underwater laser cutting was almost impossible. With the advent of fibre delivered laser power this situation has changed. This paper will describe recent work using a 5kW Yb fibre laser source, equipped with a specially designed process head for underwater laser cutting. Results will be presented for cutting both 304 stainless and C-Mn (S275JR) steel, at thicknesses of up to 32mm. The paper will also describe the various parameters thought to be important in underwater cutting and their effects on the cut achieved. It will be shown that for different applications, different sets of parameters can be used to achieve different results. As an example of this, in the application of nuclear decommissioning, for maximising dross height on the parts being cut will be described. In nuclear decommissioning, maximising the dross adhesion is important, as it means less radioactive material from the kerf is released into the water and therefore more remains on the parts being size reduced for future long term storage.