This paper presents an investigation on NOx emissions from turbulent propane diffusion flames. The study includes both experimental measurements and numerical 2D simulations and aims to provide a better understanding of the dominant physical effects associated with the NOx production in turbulent diffusion flames. Measurements of NOx emission indices (EINOx), performed in unconfined and vertical lifted flames, have been analysed numerically. In this analysis, the effects of flame radiation, flame extinction by stretch and residence time have been studied by means of a mathematical model including a subroutine of the relevant NO chemistry. The numerically calculated values of EINOx, which are in good agreement with the experimental data, are found to be proportional to the residence time of the reaction products in the high-temperature reaction zone. The similar EINOx trends with geometrical or fluid mechanical parameters obtained without taking flame radiation or flame extinction into account reveals that the residence time is the dominant physical parameter associated with NOx scaling for the present flames.