Abstract: A hydraulic-energy conversion system in which an output shaft is drivingly connected through a planetary gear to at least one turbine stage comprising a first turbine rotor with a plurality of left- or right-handed blades and a second turbine rotor with a plurality of right- or left-handed blades. In one embodiment the output shaft and those of the first and second turbine rotors are extended coaxially of each other and the sun gear carried by the output shaft is in mesh with the planet gears carried by the second turbine rotor and in mesh with the internal gear carried by the first turbine rotor. In operation hydraulic energy; that is, water or wind causes the first turbine rotor to rotate in one direction and the second turbine rotor to rotate in the other direction and because of the planetary gear transmission the output shaft can rotate about twice as fast as that of the conventional system, whereby high conversion efficiency and a high output power can be attained.