Using the nonlinear transmission (NLT) technique, the experimental investigation on. tlsrce-photon absorption processes in GaAs intrinsic semiconductor illuminated by a pulsed laser at 2.06μm is firstly described in this paper. Three-photon absorption and subsequent excited state absorption of the generated electrons and holes have been observed. Three-photon absorption coefficient has been measured and the experimental result is in good agreement with the theoretical value.