Optical dispersive nonlinearities in Er-doped optical fiber are discussed and measured at the third window wavelength 1.55μm for optical communications firstly. The experimental method, which is devel-oped by us,is based on dynamic scanning for fixed-point-interference (DSFPI) of two fiber beams. The real part and complex value of tbe third-order susceptlbility at that wavelength are also obtained from the mea-sured Kerr coefficient and nonlinear-ahsorption coefficient reported elsewhere. Optical dispersive nonlinearities in Er-doped optical fibers are discussed and measured at the third window wavelength 1.55 μm for optical communications first. The experimental method, which is devel-oped by us, is based on dynamic scanning for fixed-point-interference (DSFPI ) of two fiber beams. The real part and complex value of tbe third-order susceptlbility at that wavelength are also obtained from the mea-sured Kerr coefficient and nonlinear-ahsorption coefficient reported elsewhere.