When yttria-stabilized zirconia (YSZ) electrolyte is coated and co-sintered on top of Ni-YSZ anode support, the measured conductivities of YSZ thick films (10-30 mu m thick) are often lower than that of bulk YSZ. In this study, we found the observation by fabricating free-standing YSZ thick films and measuring and comparing in-plain and across-plain conductivities. The in-plane conductivity of free-standing YSZ film matched very well with the conductivity of mm-thick bulk sample, It was further shown that the conductivity decrease can be minimized by using better electrode morphology. Another factor that decreases the film conductivity was identified when the thick film was reduced. The conductivity decrease, similar to 26% after reduction for 1h in humidified hydrogen gas, was due to Ni-doping into YSZ during sintering process. In order to minimize the conductivity drop of thick film YSZ during SOFC (solid oxide fuel cell) operation, an intermediate layer may be used between YSZ and anode support to prevent Ni-doping during co-sintering process in addition to the well-designed electrode morphology. (c) 2006 Elsevier B.V. All rights reserved.