The electron-hole competition in doped KNSBN crystal is analyzed for the first time by measuring the recording and erasing kinetics of doped KNSBN crystals. It is shown that the shorter the response time, the sharper is the competition between electrons and holes in doped KNSBN crystals. So it leads to the net index modulation and the diffraction efficiency smaller. The reason of the hole carriers increasing has been analysed in reduced Co: KNSBN crystals. The effective carrier densities are estimated as the order of 10 15 cm-3.