The magnetic properties of double perovskites Ba 2FeNbO 6 and Ba 2MnNbO 6 were investigated. It was shown that Mn compound does not show any Jahn-Teller distortion by electron spin resonance measurements at room temperature. It was observed that formation of short range correlations that persist above T N are influenced by the variations in the average A-site ionic radius. The results reveal that the Neel temperature of all the samples is nearly the same showing that the magnetic transition temperature is insensitive to the doping in A site.