In this study, the composition of the layered perovskite SmBa0.5Sr0.5Co2O5+d was changed to different non-stoichiometric compositions by changing the substitution amount of Ba and Sr. SmBa0.45Sr0.5Co2O5+d (SBSCO-0.45/0.5) showed the lowest area specific resistance (ASR) because the area % caused by high binding energy (HBE) of O1s in the X-ray photoelectron spectroscopy (XPS) analysis was the largest compared to all other tested compositions and also the unit cell volume was smaller than that of other samples. The dense SmBa0.5Sr0.48Co2O5+d (SBSCO-0.5/0.48) showed the highest electrical conductivity. This is because SBSCO-0.5/0.48 has the smallest decrease in oxygen contents compared to other samples when subject to temperature increase. Also, through XPS analysis, it was found that the area % of the Co3+ and Co4+ coexistence regions of Co2p of SBSCO-0.5/0.48 was the largest. The electrical conductivity values and behaviors of the porous SBSCO-0.45/0.5 and SBSCO-0.5/0.48 were not significantly different. Comparing the single-cell performance with composite cathodes comprised of SBSCO-0.45/0.5 and SBSCO-0.5/0.48 with CGO91, the results show that the single-cell with the SBSCO-0.45/0.5 and CGO91 cathode showed higher maximum power density.