In the paper,The Kerr effect in single mode birefringence fiber is studied in detail. The related advantsge is discussed when it is used as fiber optic logic gate. In addition, itS theoretical analysis and calculated data as well as experimantul setup are given The " 1/0" state of intensity model fiber logic gate is defined and the pulse width of output light is measured. The experimantal resultS coincide well with theoretically calculated data.