A valve for paint ball guns includes a case for receiving a first piston rod therein in which a first end of a second piston rod is movably received. The case includes several chambers defined therein so as to introduce pressurized air into the barrel of the paint ball gun. The first piston rod includes two piston rings and one of which is larger than the other. The first piston rod is biased by a first spring. The first end of the second piston rod in the first piston rod is biased by a second spring which is easily overcome by pushing the second piston rod by a driving member. The movement of the second piston rod introduces pressurized air to apply to the piston ring of larger area to generate a force to overcome the force from the first spring and the piston ring of the smaller area.