On-line monitoring of flame stability is crucial as it is closely related to plant safety, combustion efficiency, and pollutant emissions. This paper presents a new method for the stability monitoring of a burner flame using its medial axis. Flame video clips are acquired on a combustion test rig under different combustion conditions. Each frame of the flame video clips is processed and the medial axis of the flame is extracted. The processed medial axes are then superimposed for statistical analysis. The test results show that, although the shape of the flame is randomly fluctuated even under the same combustion condition, some of the statistical shape parameters are relatively stable for the given burner condition. These parameters can be used to indicate the stability of the flame.