The structural and magnetic properties have been investigated by means of X-ray diffraction, magnetization measurements, and positron annihilation technique. The X-ray diffraction shows that the Tb2AlFe16-xMnx （x=1—8） compounds have a hexagonal Th2Ni17-type structure. The positron annihilation spectra show that Mn substitution for Fe has the ferromagnetic interaction decreased, and that there exists a strong magnetoelastic effect in Tb2AlFe16-xMnx compound. The magnetization measurement shows that the Curie temperature and the spontaneous magnetization of Tb2AlFe16-xMnx compounds decrease rapidly with increasing x.