Abstract: The present invention relates to an improved uni-directional meter prover apparatus which includes a prover barrel having first and second ends, a piston disposed within the barrel which is propelled through the barrel by the flow of liquid therethrough, means for detecting the passage of the piston and comparing the volume of liquid displaced thereby with the volume of liquid measured by a meter being proved and means for transferring the piston from the second end of the prover barrel to the first end thereof. By the present invention, the first end of the prover barrel is positioned below the second end thereof and a piston transfer conduit is connected between the first and second ends of the prover barrel. First and second valve means are disposed within the transfer conduit for preventing liquid communication between the first and second ends of the prover barrel and for selectively retaining the piston therein.