The effect of gas flame annealing on the structure of polycrystalline Si films was studied using transmission electron microscopy and electron spin resonance. This annealing technique involved heating the sample surface to more than 1100 °C using flat gas flames with a scan rate of 1 mm/s and a heating rate of about 260 °C/min. Electron microscopy images revealed that the secondary grain growth proceeded with increasing the number of annealing times (annealing frequency) and that the grain size for samples annealed at 1360 °C was more than 1 &mgr;m whereas secondary grain growth was not significant for samples annealed at 1150 °C. Further, it was found that the spin density in the samples annealed at 1360 °C decreased from 1.5×1018cm−3to 3.8×1017cm−3. It was concluded from the spin resonance results and the electron microscopy images that the secondary grain growth consists of two processes, the initial structural rearrangement of the grain boundaries and the subsequent grain growth. ©1995 American Institute of Physics.