Ultrafast time-resolved pump-probe (TRPP) apparatus has been applied to study the carrier dynamics of Si-doped InN thin films grown buffer by molecular beam expitaxy with and without a low-temperature growth GaN buffer layer. The peak of the PL has been found to increase from 0.7 to 0.8 eV with the back ground density. The total decay rates as a function of the delay time were obtained by the density-dependent TRPP peak intensity and the time-resolved TRPP signals. The total decay rates were interpreted as the sum of radiative and nonradiative recombination. The Shockley-Read-Hall decay rate derived from the TRPP signal at low photoexccitation density was found to increase with the doping density. At low concentration, the Auger recombination is not effective. The dominant recombination mechanism at room temperature is the Shockely-Read-Hall recombination.