Layered perovskite cobalt oxides Sr2−yYyCoO4 y=0, 0.1, 0.3, 0.5, 0.67, 0.83, and 1 were synthesized under high pressure and high temperature conditions. Structure refinement revealed that these compounds crystallize in K2NiF4-type structures with space group I4/mmm. The parent compound Sr2CoO4 undergoes a ferromagnetic transition with Tc=255 K. The Tc decreases with increasing y to 150 K for y=0.5, and ferromagnetism was not observed for y0.67. Assessment of spin states for Co3+ and Co4+ ions suggested strongly that both are present as intermediate spin states when y0.67 at least for the higher temperature range above Tc. Fairly large negative magnetoresistance was observed for Sr2CoO4 in the vicinity of Tc and in the lower temperature region.