A fastener driving tool includes a striking member and an energy storing unit disposed in a housing. The energy storing unit has at least one torsion spring having forwardly extending short and long legs which are connected to a pivotable handle and the striking member. An anchoring member is disposed forwardly of and slidable relative to the handle and is detachably engaged with the striking member. During a pivotal movement of the handle, the short and long legs of the torsion spring are moved in opposite directions to store a biasing energy and to force the striking member upwards, which in turn moves the striking member downward for performing a fastener striking stroke once the anchoring member is disengaged from the striking member to release the torsion spring.