Abstract: 6 kW disk laser-oxygen cutting experiment on 30 mm thick steel plate was carried out to reveal the thick plate laser cutting mechanism. The influence of main process parameters on laser cut quality was investigated. The results indicated that two laser cutting modes, i.e., narrow kerf mode and wide kerf mode were found when performing thick plate laser cutting. Oxygen pressure and laser defocused distance had a significant influence on transformation between two laser cutting modes. Cross section of wide kerf was found to be V type, while narrow kerf is I type. The kerf surface of two laser cutting modes was observed, indicating two different laser cutting processes.