Degenerate pump-probe experiments have been performed on the dynamics of carrier and phonon in GaInNAs thin films. The time-resolved differential transmission shows a negative value, indicating photoinduced absorption from the trap states. After the negative minimum the differential transmission recovers to zero with a long time constant. Rate equation has been employed to simulate the carrier dynamics. The calculations fit the experimental differential transmission very well. The extracted time constants show that the carriers in the trap states of GaInNAs decay to equilibrium with a single time constant of 1.2 ns. An obvious modulation of the transmission signal has been observed superimposing on the photoinduced absorption. Such a modulation is found at a low frequency of 380 GHz by Fourier transform. This low frequency oscillation might be attributed to coherent longitudinal acoustic (LA) phonon.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.