Using the reflective measurements, the effect of grain boundaries on the lifetime of photogenerated carriers was investigated for chemical-vapor-deposited (CVD) diamond films. To investigate the effect of grain boundaries on the carrier dynamics, photons having energy lower than the band gap energy of a single crystal of diamond were used for pumping. For a 4.8 eV photon, the measured lifetime was several tens of picoseconds, and that was consistent with photoconductive current measurements. However, a dramatic decrease of the carrier lifetime was observed in the case of 3.2 eV irradiation. The variation of the lifetime inside the single grain was measured by a microscopic pump–probe method. The carrier lifetime near the grain boundary decreased from 5 to 8 ps at the center to 0.35–0.5 ps. This decreased lifetime and the carrier generation efficiency with lower energy photon had a negative correlation. To explain this mechanism, we considered the decrease in lifetime to be related to the density of the imperfection or mid-band gap states inside the single-CVD-polycrystalline grain. © 2000 American Institute of Physics.