NO production is simulated and analyzed in three lean H-2/air premixed flames: the classic one-dimensional (1D) flame, the opposed jet flame, and the tubular flame. The 1D flame is simulated with the PREMIX code followed by the SENKIN code to determine the NO production for a long residence time. The opposed jet flame and the tubular flame are simulated with the OPPDIF and TUBEDIF codes, respectively. The major factors influencing NO production are flame temperature, radical concentration, and post-flame residence time. For the 1D flame, almost 100% of NO is produced in the post-flame region and the reactant residence time in this region is the only factor determining NO concentration. For the opposed jet flame, flame stretch influences all three factors and the competition among them controls the NO concentration response to stretch rate. For the tubular flame, flame curvature influences flame temperature and post-flame residence time, and it makes the NO concentration response different from that of the opposed jet flame.