Abstract: The flickering of laminar diffusion flames is a typical flame instability phenomenon; yet, a comprehensive understanding of the various instability modes and their frequency characteristics remains insufficient. This study aims to bridge this gap by conducting an experimental investigation on the instability and frequency characteristics of buoyancy-driven and momentum-driven round laminar jet diffusion flames in a wide range of flame conditions, by means of simultaneous high-speed flame chemiluminescence and particle image velocimetry. Upon increasing the fuel jet flow rate, the flame becomes unstable and starts to flicker, exhibiting two distinct modes: varicose and sinuous modes. The flow field measurement results indicate that the primary flow structures of flickering jet diffusion flames consist of two shear layers, located inside and outside the flame surface, respectively. And the periodic generation, growth, and shedding of large-scale toroidal vortices, formed by the rolling up of the outer shear layer, is the primary cause of the periodic deformation of the flame surface, i.e. flame flickering. The flickering flame displays quasi-periodicity, with a unique dominant frequency observed in the flow field. This frequency is found to be consistent with the frequency obtained from the spectrum analysis of the flame chemiluminescence fluctuation, underscoring that flame flickering is essentially a manifestation of global hydrodynamic instability. Moreover, it is noteworthy that the sinuous mode of buoyancy-driven flame exhibits a frequency approximately 3 Hz higher than that of the varicose mode. Additionally, at a larger fuel flow rate, there is an increased tendency for flame transition between the varicose and sinuous modes. The frequency of buoyancy-driven flames accords with the classical 1/2 scaling law, with different modes corresponding to different scaling law coefficients. The frequency of momentum-driven flame significantly deviates from the 1/2 scaling law, indicating that the specific flow instability modes have a crucial influence on the frequency characteristics of flickering jet diffusion flames.