The unsteady effects of buoyancy-induced instabilities on jet diffusion flames are investigated experimentally under normal gravity conditions. Methane and propane are used as test fuels that are lighter and heavier than ambient air, respectively. A similar Froude (Fr) and Reynolds (Re) number relationship is realized in both hydrocarbon fuels with different tube diameters ranging from 6 to 24.2 mm. The Schlieren visualization technique and high-speed imaging synchronized with chemiluminescence signal measurement are used to identify changes in global flame shape and dominant frequency. Buoyancy-induced instabilities generate two forms of diffusion flames with varying frequencies in space. Both laminar and turbulent jet flames exhibit natural and subharmonic frequencies, as well as a shift between them. The methane-propane Re–Fr relationship confirms the instability mode transition. In addition, Strouhal (St) and Froude number relations are obtained as St∝Fr-0.50, with a slope difference between natural and subharmonic modes in both fuels. © 2021, The Author(s), under exclusive licence to Springer Nature B.V.