We present a time-resolved ultrafast measurement in terahertz (THz) frequency region by means of the free-space electro-optic sampling. The fast delay scan technique is used to suppress the noise with low frequency and to improve the signal-to-noise ratio of the system. The transmission spectra of different materials are obtained.The optical properties of these materials in a THz region are shown. The broadening of spectrum and chirping phenomena are illustrated. We find that polystyrene is an excellent material for the THz application.