In the metal filament formation-based organic memory, the positive voltage application over the threshold electric field strength (170 MV/m) is necessary for the filament formation in Cu/P3HT/Al device. By the positive voltage application, the copper ions are generated and drifted into polymer layer, which is clearly confirmed by the secondary ion mass spectroscopy. Also, the field strength (100 MV/m) required for the drift process could be independently determined with a new pulse operation method. We could conclude that the threshold field strength of 170 MV/m was determined by the ionization process of copper. Furthermore, the dependence of the positive field strength and the temperature on the memory behavior was studied.