Experimental studies on the coupled effects of transport properties and unsteady fluid dynamics have been conducted on laminar, acoustically forced, hydrogen jet diffusion flames diluted by argon and helium. The primary purpose of this research is to determine how the fuel Lewis number and the flow unsteadiness play a combined role in maximum flame temperature and affect NOx emission from jet diffusion flame. The fuel Lewis number is varied by increasing/decreasing the mole fraction of diluents in the fuel stream. Therefore, maximum flame temperatures and then NOx emission levels were expected to differ for Ar- and He-diluted flames. In an investigation of unsteady flames, two different frequencies (10 and 100 Hz) were applied to observe a behavior of NOx emission levels and flame lengths by changes of unsteady fluid dynamics and transport properties.