T91 ferritic/martensitic (F/M) steel is an expected structural material candidate for Gen IV liquid lead-bismuth cooled nuclear reactors. However, molten lead-bismuth eutectic alloy (mLBE) often causes liquid-metal embrittlement (LME) of F/M steels. Although prior austenite grain boundaries and martensite block boundaries were reported to be preferential sites for LME, the mechanism of LME in a T91/LBE couple is yet to be comprehensively understood. In this paper, the effect of mLBE on T91 steel was investigated using micropillar compression tests. mLBE corrosion was found to cause a significant decrease in yield stress. The micropillar made from mLBE-corroded specimen was significantly sheared along high angle boundaries. In addition, EDS analysis demonstrated the presence of Pb/Bi at these high angle boundaries.