We have tested Time-Of-Flight detectors consisting of several combinations of scintillators and photomultiplier tubes for the identification of produced heavy ion beam. Timing resolution of 6.8ps (RMS) is achieved with energy deposited of 150-160MeV in the plastic scintillator, where 4 0 Ar beam is used as a primary beam at the total energies of 3.8GeV. Our systematic studies suggest the importance of maximizing the number of photoelectrons, where the timing resolution seems to be scaled by the pulse height according to the characteristic function of the photomultiplier tube. In addition, scintillator with faster timing property seems to be also essential for better timing resolution.