The photoluminescence behavior of (Y1-xTbx)PO4 phosphor was investigated by measuring emission and excitation spectra, and decay curves. The relative emission intensity and the decay time have been monitored as a function of Tb3+ concentration for both D-5(4) - F-7(j) and D-5(3) - F-7(j) transitions. The decay behavior of D-5(4) - F-7(j) transition, for which well known cross-relaxation has been accepted as a main factor was analyzed by direct quenching mechanism on the basis of dipole-dipole interaction. It is. however, impossible to characterize the D-5(4) - F-7(j) transition using the direct quenching scheme. In fact, the decay curves of D-5(4) - F-7(j) transition could be analyzed by intercenter energy migration. Another type of cross-relaxation was suggested to explain the quenching of D-5(4) F-7(j) transition.