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We investigate transmission characteristics and sheet conductivity of mono-to multi-layer graphene deposited on quartz in the terahertz (THz) frequency region. The free carrier absorption and Fabry-Perot interference between graphene layers give rise to nonlinear decrease of THz transmission from 76.7% to 27% for mono-to 12-layer graphene. These phenomena are well explained with a modified theoretical model based on Drude conductivity. The optical sheet conductivity of multi-layer graphene, made by layer-by-layer random stacking of high-quality mono-layer graphene, at 1 THz exhibits two orders of magnitude higher values than the universal optical conductivity due to intraband transition of intrinsic graphene.