Abstract: Hydrogen and ammonia have attracted increasing attention as carbon-free fuels. Ammonia is considered to be an effective energy storage and hydrogen storage medium. However, a small amount of unremoved NH3 is still present in the product during the decomposition of ammonia to produce hydrogen. Therefore, it is very essential to investigate the self-ignition of hydrogen-ammonia mixtures in order to accommodate the various scenarios of hydrogen energy applications. In this paper, the effect of NH3 addition on the self-ignition of high-pressure hydrogen release is numerically investigated. The RNG k-ε turbulence model, EDC combustion model, and 213-step detailed NH3/H2 combustion mechanism are used. CHEMKIN-Pro programs for zero-dimensional homogeneous and constant volume adiabatic reactor models are used for sensitivity analysis and ignition delay time of the chemical reaction mechanism. The results showed that the minimum burst pressure required for self-ignition increased significantly after the addition of ammonia. The maximum temperature and shock wave intensity inside the tube decreases with increasing ammonia concentration. The ignition delay time and H, HO2, and OH radicals reduce with increasing ammonia concentration. H and HO2 radicals are suggested as indicators for tracking the second and third flame branches, respectively.