The radiation effect on flame temperature and NO emission of H-2-lean (0.2H(2)+0.8CO) and H2-rich (0.8H(2)+0.2CO) syngas/air counterflow diffusion flames was numerically investigated using OPPDIF code incorporated with the optical thin model, statistical narrow band model and adiabatic condition. Firstly, the coupled effect of strain rate and radiation was studied. Disparate tendencies of NO emission with an increasing strain rate between H2 lean and H-2-rich syngas flames were found at very small strain rate, and the effect of radiation reabsorption on NO formation can be neglected when the strain rate was greater than 100 s-1 for both H-2-lean and H-2-rich syngas flames. Because the radiation effect is vital to flames with small strain rate, its impact on flame temperature and NO emission was investigated in detail at a strain rate of 10 s(-1). The results indicated that NO formation is more sensitive to radiation reabsorption than flame temperature, especially for the H-2 rich syngas flame. The underlying mechanism was discovered by using reaction pathway analysis. Furthermore, the radiation effect under CO2 dilution of the syngas fuel was examined. It was demonstrated that the radiation effect on flame temperature became more prominent with the increase of CO2 concentration for both H-2-lean and H-2-rich syngas. The radiation effect on NO emission increased first and then decreased with an increasing CO2 content for H-2-lean syngas, whereas for H-2-rich syngas the radiation effect is monotonic. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.