Reflection-type degenerate pump-probe spectroscopy was performed for low-temperature grown (LT-) GaAs to study the effects of arsenic pressure during crystal growth and annealing on carrier lifetime and to ascertain the annealing dynamics. It was found that a sample grown under a high arsenic pressure has a shorter carrier lifetime for both as-grown and anneal conditions. It was also found that the carrier decay times of samples changed drastically when the annealing temperature was above 550 °C. We determined the annealing dynamics of LT-GaAs based on a model in which AsGa antisite defects trap photoexcited carriers. An Arrhenius plot of the carrier decay rate vs. annealing temperature in the high temperature regime gave an energy EPA that was different from the true activation energy. The annealing time dependences of EPA obtained by the two diffusion models (self diffusion and VGa vacancy assisted diffusion of defects) were compared with EPAs of our data and other works, which proved that the annealing dynamics of AsGa antisite defects was dominated by VGa vacancy assisted diffusion.