Based　on　the　principle　of　multiple-reflection　in　white-light　interferometry,　a　new　optical　fiber　biosensor　is　investigated.　An　optical　fiber　immobilized　with　protein　molecule　(antigen　or　antibody)　is　adopted　as　a　high　sensitivity　biosensor　to　accomplish　the　real　time　monitor　of　the　immunoassay.　The　experimental　results　of　rabbit-IgG　with　anti-rabbit-IgG　immunoreaction　show　that　by　the　aid　of　this　biosensor,　a　biolayer　change　of　approximately　0.70　nm　in　optical　thickness　can　be　detected　by　measuring　the　reflected　spectrum　phase　shifting　between　the　two　interfaces　of　biolayer.　Compared　with　the　others,　this　method　has　advantages,　such　as　simple　structure,　lower　cost,　high　sensitivity　and　reliability,　label-free　in　immunoassay　and　direct　monitor　the　immunoassay.　This　biosensor　is　easy　to　be　integrated　as　a　BIAcore　(biomolecular　interaction　analysis　core)　for　parallel　direct　detection　of　antigen-antibody　interactions.