We report on state-of-the-art JÜLICH (Forschungszentrum Jülich) stack with anode-supported solid oxide fuel cells (AS-SOFCs) that have been tested in bio-syngas derived from wood pellets. The sulfur and chlorine were removed after gasification, but the tars were not reformed in the bio-syngas to study the influence of these tars on the degradation of SOFC stack. The total tar content during test was 3.5 g/Nm3 including benzene, toluene, phenol, m-cresol, naphthalene, and minor traces of undefined tars. The test result shows considerable performance degradation in tar-contaminated syngas. Moreover, the test was stopped after 5 hours of operation due to an increase of the pressure drop in the stack. A post-test analysis was carried out, and heavy carbon deposition was found at the cell anode-support surface and the Ni mesh current collector. Carbon was identified by SEM as numerous carbon fibers. The change of support microstructure was also observed near and under the carbon deposition area, and the dusting of Ni metal was observed in the support and Ni mesh current collector.