We investigated carrier-relaxation dynamics of femtosecond laser annealed (FLA) polycrystalline silicon (poly-Si). The correlation between morphology and electrical properties of poly-Si after femtosecond laser annealing is elucidated by optical-pump-terahertz-probe and terahertz time-domain spectroscopies. The transient conductivities of FLA-processed poly-Si with large (∼500 nm) and small (∼50 nm) grain sizes were both well fitted by the Drude model in the terahertz regime from 0.4 to 2 THz. The transient mobilities of these materials were determined to be 175±19.4 and 94.5±20.2 cm 2/V∈s, respectively. After annealing, reduction of deep-state density rather than tail-state density in large-grain poly-Si is responsible for its higher mobility.