This article reports an experimental study on the effect of the internal geometry of a circular nozzle on the stability of a turbulent methane diffusion flame with and without co-flow. The nozzle exit orifice is circular; however, the orifice diameter, length-to-diameter ratio, and contraction angle were varied. These geometrical parameters were aimed to create a wide range of conditions for the ensuing jet flow. The strength of the co-airflow was also varied to assess its impact on the jet flame stability parameters. The experimental results showed that the level of turbulence in the jet near field has a definite impact on the liftoff (i.e., local quenching) of an attached jet methane flame. This is obtained by systematically varying the nozzle geometry, where these variations led to the same conclusion that higher levels of jet near-field turbulence results in a lower flame liftoff velocity regardless of the nozzle geometry. The present results revealed also that there is a clear interplay between the flame liftoff height and the jet flow characteristics. That is, a jet flow that spreads faster and generates greater near-field turbulence would result in a flame base sitting closer to the nozzle exit. Finally, the presence of a moderate co-airflow results in the appearance of a hysteresis phenomenon at low jet velocity range but, in general, did not alter the relationship between the flame stability parameters and jet's characteristics.