A planar heat pipe has a hollow planar tube and a wick structure attached to an interior sidewall of the planar tube. A spiral support member is installed to extend along the interior sidewall, so as to press the wick structure against the interior sidewall. The spiral support member has a plurality of interstices extending laterally within the planar tube. A planar support member is inserted between the spiral support member in the hollow planar tube. The planar support member has a plurality of voids extending along an elongate direction of the planar tube. Thereby, cross flowing channels are formed inside of the heat pipe.