The crystal structure and magnetic properties of perovskite-related (Tb(subscript 1-x)Na(subscript x))MnO(subscript 3-y) compounds in the compositional range 0≤x≤0.3 are reported. We found that the effective moments decrease with increasing Na content, which is due to weaken the rare-earth interaction and dilute average rare-earth moment in each unit cell. Moreover, a decrease of unit cell volume with Na-doping is also observed resulting from an increase in the oxygen vacancies. The result is supported by the X-ray absorption near-edge structure (XANES) spectra and the closed-ampoule iodometry technique.