The basic characteristic of electric field is that the substance in the field is in operation by the stress. Under steep pulsed electric fields, there is strong distribution of electric fields on the inside and outside surfaces of cell membrane. In virtue of the difference of the permittivity among membrane, cytoplasm and extracellular medium, there must be the electric field stress on the surface of the membrane. The calculation results showe that cell membrane is stressed from two directions of cytoplasm and extracellular medium under external electric fields. The stress on tumor cell membrane is greatly larger than normal cell membrane, so the tumor cell is more sensitive than normal cell. Tumor cell membrane certainly will be destroyed by electric field stress, then its survival environment and germ plasm will be injured and accordingly the cell will be damaged.