Magnetic and transport studies of the compounds Nd _0.7 Sr _0.3 Mn _1-x Co _x O ₃ (0.0≤x≤1) have been carried out. The compositions crystallize in single-phase ortho-perovskite. The conduction mechanism could be explained using the small polaron correlated hopping model. The thermal irreversibility in zero-field and field-cooled magnetization data increases with Co content, which is due to the different spin states of the Co. Transition of Co ³⁺ from high spin/intermediate spin to low spin state is observed at low temperatures. The nonsaturating M-H behavior is observed in cobalt containing samples due to suppression of double-exchange interaction which favors long-range ferromagnetic order.