Abstract: This paper reveals the selective injury effect and suppressing growth effect of low frequency magnetic field on tumor cells. The effects occur in special `frequency windows' and `power windows'. This paper also points out the sensitization from low frequency magnetic field occurs in radiotherapy and chemotherapy of tumor and the multi-drug resistance is decreased, analyzes the cause of physical sensitization. Finally, this paper presents that the low frequency pulse magnetic field should be normalized for the uncertainty from differences of pulse magnetic field.