Quantum frequency conversion,1 a process with which one beam of light can be converted into another of a different frequency while preserving the quantum state, is experimentally demonstrated. An implementation of quantum frequency conversion requires that an input beam to a sum-frequency generator be completely up-converted. One needs to generate a quantum state (e.g., a squeezed state or a twin- beam state) at one frequency and then show that the properties of the same state appear at the up-converted frequency. We have chosen to use nonclassical intensity correlation of the twin beams as the input quantum property because large correlations can be easily obtained by pumping an optical parametric amplifier with a mode-locked and Q-switched laser.2 After one of the twin beams is up-converted, nonclassical intensity correlations appear between the up-converted beam and the remaining twin beam. In our experiment the 1064nm output signal beam of the parametric amplifier, possessing >3 dB nonclassical intensity correlation with the 1064nm output idler beam, is up-converted into a 532nm beam. The 532nm beam shows >2 dB nonclassical correlation with the 1064nm idler beam, demonstrating the quantum frequency conversion process.