This paper investigates the effects of coflow O

2

level and temperature on diffusion flame of a CH

4

/H

2

jet in hot coflow (JHC) from a burner system similar to that of Dally et al. The coflow O

2

mass fraction (y

O

2

*

) is varied from 3% to 80% and the temperature (T

cof

*

) from 1200 K to 1700 K. The Eddy Dissipation Concept (EDC) model with detailed reaction mechanisms GRI-Mech 3.0 is used for all simulations. To validate the modeling

several JHC flames are predicted under the experimental conditions of Dally et al. [Proc. Combust. Inst.

29 (1)

1147-1154 (2002)

]

and the results obtained match well with the measurements. Results demonstrate that

when y

O

2

*

decreased

the diffusion combustion is likely to transform from traditional combustion to MILD (Moderate or Intense Low-oxygen Dilution) combustion mode. When T

cof

*

is higher

the temperature distribution over the whole domain trends to be more uniform. Reducing y

O

2

*

or T

cof

*

leads to less production of intermediate species OH and CO. It is worth noting that if y

O

2

*

is high enough (y

O

2

*

80%)

increasing y

O

2

*

does not cause obvious temperature increase.