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A series of Th and F co-doped superconductors Sm1-xThxFeAsO1-yFy are synthesized and the variation of superconductivity with the doping level is investigated. At the fixed Th doping level x = 0.1, the superconducting transition temperature Tc increases monotonically with F content, and finally Tc reaches a maximum of about 55K around y = 0.1, and saturates for even more F content (y = 0.15). Similar to the SmFeAsO1-yFy system, the normal state thermopower increases monotonically with the doping level. However the decrease of Tc in the overdoped regime is not observed and possible explanation is discussed.