The effect of metal sulfides in the cathode on Na/S battery performance was investigated. Battery capacity was heavily decreased by doping with FeS2, CoS2 and Cr2S3, and little influenced by doping with NiS2 and Al2S3. A FeS2 3.0wt.% doped cell exhibited a 65% resistance rise and a 65% capacity loss during cycling. Such a degradation causing by the metal sulfides was mainly attributable to a low charge acceptability of the cathode and closely related to the cathode structure.