Abstract: The preheating amplitudemodulated theory model applied in the lower ionosphere modulated heating was constructed based on elementary electron energy equation and continuity equation. Then, extremely low frequency/very low frequency (ELF/VLF) radiation levels using preheating amplitudemodulated mode in the ionosphere amplitude modulated heating was studied in detail. Taken HAARP heater for example, we compared ELF/VLF dipole moment magnitude produced by general amplitude modulated heating mode with preheating amplitude modulated mode. The results indicate that when the radiated power of heater system higher and of ELF/VLF frequency lower, the preheating amplitude modulated mode is more effective. Under certain heating conditions, ELF/VLF radiation levels improve by 5.3 dB using preheating amplitudemodulated mode comparing to general amplitudemodulated mode.