The crystallization process of PZT thin films has been studied in situ by means of Environmental Scanning Electron Microscopy ( ESEM ). Based on the ESEM observations, the Rapid Thermal Processing (RTP) technique has been employed to crystallize ferroelectric thin films. Various annealing temperature-time combinations were investigated; the results indicate the crystallization process to be very fast. X-ray diffraction data shows the crystallization to be complete in 10 seconds at 600°C, and in 1 second at 700°C. In comparison with conventionally furnace processed films, the RTP films have comparable ferroelectric and dielectric properties, but are distinguished by superior breakdown strengths and morphologically smoother surface. The relation between microstructure of films and crystallizing parameters has been studied.