Abstract This paper presents numerical studies of flame structure in a well-known nonpremixed H2/N2 jet flame. A detailed kinetic mechanism is implemented to large eddy simulation approach with linear eddy model as subgrid closure method. The numerical techniques are validated against experimental data in temperature profiles, specie mass fractions and mixture fractions. Reaction rates are calculated at the flame base to investigate the dominant chemistry for a variety of conditions of fuel and coflow. The results show that the decrease of hydrogen concentration leads to chemical pathways changing at the flame base that is responsible for the autoignition. Oxygen content variation in the coflow affects the reaction rates but does not change the dominant chemistry. Furthermore, water addition to the coflow is found to enhance NOx formation by promoting radical production.