A model scramjet engine in which the 1.0 Ma hydrogen jet mixes and reacts with the 2.0 Ma surrounding airstream is investigated using large eddy simulation. The flame structure is analyzed with a focus on the relationship between premixed/diffusion combustion mode and heat release in the supersonic reacting flow. The flame filter is used to evaluate the contributions to heat release rate by different combustion modes qualitatively and quantitatively. Results show that the heat is released from a combination of premixed combustion mode and diffusion combustion mode even when the fuel and airstream are injected into the combustor separately. Local mode-transition occurs as the supersonic jet flame propagates and interacts with shocks. The diffusion combustion mode dominates during the ignition stage and the premixed combustion becomes dominant during the intensive combustion region. When the shock wave impinges on the flame, the combustion area decreases a little due to the compression effects of the shock. However, the heat release rate is significantly improved in the interaction region since the shock could increase the air entrainment rate by directing the airflow toward the fuel jet and enhance the mixing rate by inducing vorticity due to baroclinic effects, which is good for flame stabilization in the supersonic flow. For the present case, 33.3% of the heat is released by diffusion combustion and 66.7% of the heat is released by premixed combustion. Thus the premixed combustion mode is dominant in terms of its contributions to heat release in the model scramjet engine. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.