The hydrodynamic instabilities in the shear layer of fuel jet leads to transition of gas jet diffusion flames even when the injector flow is laminar. The current study investigates the transition process and identifies the dominant effect of buoyancy on the transitional flame structure. A highspeed digital imaging system was integrated with the rainbow Schlieren apparatus to visualize hydrogen jet diffusion flames in quiescent air at image acquisition rate of 2000 frames per second and exposure time of 23 μs. The flame visualization revealed that the well known buoyancy driven instability is accompanied with secondary roll up vortices on the flame surface, with similarity in structure to the Kelvin Helmholtz instability. The frequency spectra analysis revealed that the flicker, typical of laminar flames, also occurs in transitional flames. The fuel jet also oscillates at the flame flicker frequency caused by the bulging and squeezing effect of buoyancy driven outer vortical structures. The fuel jet oscillations lead to intermittency in the breakpoint length. The period oscillations are sustained even in the turbulent fuel jet and flame regions downstream of the breakpoint.