The ultrafast photo-conductive characteristics of LT-GaAs were studied with femtosecond photocurrent correlation measurements. A response time ranging from 350 to 390 fs of a LT-GaAs micro-coplanar-strip-lines gap for different exciting photon energies and different bias voltages was obained. It has been estimated that the transport rate of the photo-excited electrons in the gap is 1000cm²/V^.s. The experimental traces are well consistent with a theoretical normalized transient photo-current correlation function.