A windmill includes a plurality of fan blade units pivotally mounted in a central vertical transmission shaft each unit having a right-leaf vertical fan blade and a left-leaf horizontal fan blade in which the vertical fan blade as rotated to the leeward can be feathered by a forward rotation of a driving motor to reduce its wind resistance during its leftward rotation towards the windward, and the horizontal fan blade when rotated to the windward can be vertically erected ready for receiving wind energy by a reverse rotation of the motor during its righward rotation towards the leeward for efficiently collecting the wind energy for driving the transmission shaft for power generation.