The femtosecond visible pump mid-IR probe technique was employed to investigate photogenerated carrier dynamics in n-GaAs (100) crystals Subjected to various surface treatments in a time domain of picoseconds-several tens of picoseconds. Pt- or Ru-treatment significantly accelerated the carrier recombination as a result of the introduction of surface states. The recombination rate was also increased by Au-treatment but less significantly than that by the Pt-or Ru-treatment. The effect of sulfide treatment on the recombination dynamics seemed to be rather small, although it has been reported that the density of Surface states is decreased by the Sulfide treatment.