We have performed magnetization measurements and electron spin resonance (ESR) on polycrystalline manganites of Nd0.5Sr0.5-xBaxMnO3(x=0.1). Phase separation and phase transitions are observed from the susceptibility and the ESR spectra data. Between 260K (～TC) and 185K (～Tn), the system coexists of the paramagnetic phase and the ferromagnetic (FM) phase. Between 185K and 140K, the system coexists of the FM phase and the antiferromagnetic (AFM) phase. These results indicate that the system has a very complex magnetic state due to the origin of the instability stemming from manganite Nd0.5Sr0.4Ba0.1MnO3 by partially substituting the larger Ba2+ ions for the smaller Sr2+ ions.