The dilution effects of CO2 and H2O on partially premixed swirling syngas flames are investigated with the large eddy simulation (LES) method. The linear-eddy model (LEM) is employed to directly resolve the unclosed molecular diffusion, scalar mixing and chemical reaction processes occurring at subgrid scale level using their specific length and time scales instead of modelling, which makes the LES-LEM approach quite attractive for hydrogen fuel combustion as the obviously different diffusion and reaction characteristics of H2 and H compared to other species in the syngas mixture. Firstly, adding CO2 into the fuel stream can significantly decrease the flame temperature during the partially premixed combustion. The concentration of H and OH radicals decreases upon CO2 dilution and thus the chemical reaction processes are modified. Compared with CO2, H2O is less effective in changing the temperature field because of the chemical effects of H2O. The simultaneous addition of H2O and CO2 as dilution gases with volume ratio 1:1 into the fuel stream is also conducted to identify the effects of H2O and CO2 on partially premixed combustion dynamics by comparing with single H2O and CO2 cases. The obtained results are expected to provide helpful information for the design and operation of gas turbine combustion systems with syngas fuels. © 2017 Energy Institute