A flat heat pipe with a composite capillary structure has a flat pipe with a flat and enclosed hollow pipe body including a top wall, a bottom wall, two lateral walls and a chamber. The flat pipe has an evaporation section and a condensation section. The elongated mesh grid is located onto either of the top and bottom walls in the chamber. The elongated mesh grid is extended from the evaporation section to the condensation section. The long porous sintered structure is located adjacent at least one lateral wall in the chamber. The long porous sintered structure is extended from the evaporation section to the condensation section. The porous sintered structure and the elongated mesh grid are prefabricated into a composite capillary structure. The flat heat pipe presents excellent diversion effect and stable positioning with its better vapor diversion space and simple manufacturing process.