SrBi2(Ta1-xNbx)1.95M 0.05O9 (x = 0~1;M=W and Mo) were investigated from the viewpoint of ferroelectric properties, crystal and electronic structures. By P-E hysteresis-loop and dielectric-constant measurements, it was demonstrated that both the W- and Mosubstituted samples exhibited higher remanent polarization and lower Curie temperature regardless of the Ta:Nb ratio. The Rietveld technique with the neutron diffraction data indicated that the tilting angles of the (Ta,Nb,M)-O6 octahedra and the spontaneous polarization increased by the W and Mo substitutions. Such a change in the crystal structure explained well the behavior of the ferroelectric property. From electron-density analysis using synchrotron X-ray diffraction patters, it was suggested that the bond strength within the (Ta,Nb,M)-O6 octahetron became weaker slightly by the partial replacements of W and Mo.