  The Fine Pixel CCD (FPCCD) is one of the candidate sensor technologies for
the ILC vertex detector. The vertex detector is located near the interaction
point, thus high radiation tolerance is required. Charge transfer efficiency of
CCD is degraded by radiation damage which makes traps in pixels. We measured
charge transfer inefficiency (CTI) of a neutron irradiated FPCCD prototype. We
observed a degradation of CTI compared with non-irradiated CCD. To improve the
CTI of irradiated CCD, we performed the fat-zero charge injection to fill the
traps. In this paper, we report a status of CTI improvement.
