Understanding and optimizing electrochemically active zone for oxygen reduction reaction in solidoxide fuel cells (SOFCs) cathodes are indispensable to maximize device’s performance. For mixed ionicand electronic conductors, the range of this zone depends on the oxygen surface exchange and solid-stateoxygen/electron transport properties of materials [1]. This aspect is also critical to realize validcharacterization of the electrode’s electrochemical activities. For instance, when a material’s electronicconductivity is low or the electrical contact points are spaced beyond a critical distance, the electrodepolarization resistance measured by electrochemical impedance spectroscopy will be controlled by the sheetresistance not the oxygen exchange resistance [2]. The situation becomes more complicated as the electricalconductivity of porous layer, which is an advantageous structure with large surface area and facile gas phasetransport, considerably deviates from the material’s intrinsic property. It is expected that porosity,connectivity, and impurity contents can have significant impact on porous electrode’s electronic conduction[3].In this context, we have studied effective in-plane electronic conductivity of lanthanum strontiumcobalt ferrite (LSCF) porous layers. The in-plane conductivity was measured by four-line probeconfiguration [4], modified from the widely used four-point probe technique to be applied for a porouslayer. We have confirmed that the in-plane electrical conductivity of screen-printed LSCF porous layers isone or two orders of magnitude lower than the material’s bulk property in ambient air, depending ontemperatures. This electronic conductivity deviation was taken into account to rationally designcombination of electrode thickness and current collector spacing and, in turn, to guarantee a validassessment of electrode performance. In addition, we found that electrical conductivity of LSCF porouslayers decreases by 20~30 %, within relatively short period of time (250~400 hours) upon repeated thermalcycles (500 ℃ to 700 ℃). Furthermore, we found that in-plane conductivity of thin (<10 µm) electrodes isstrongly dependent on the measurement time and the local atmospheric environment relative to thickerelectrodes. Special attention is required to use a thin electrode to evaluate electrode performance.This work was supported by the US Department of Energy (DOE), Office of Fossil Energy, (Officeof Clean Coal & Carbon Management), “Solid Oxide Fuel Cell Manufacturing in Support of Office ofFossil Energy” program through Argonne National Laboratory under FWP No. 27327.1.[1] S.B. Adler et al. J. Electrochem. Soc. 143 (1996) 3554-3564; S.B. Alder Solid State Ionics 111 (1998)125-134.[2] B.A. Boukamp et al. Solid State Ionics 192 (2011) 404-408; B.A. Boukamp et al. Solid State Ionics 283(2015) 81-90.[3] L. Holzer et al. J. Mater. Sci. 48 (2013) 2934-2952.[4] S.W. Peterson et al. J. Electrochem. Soc. 161 (2014) A2175-A2181.