Abstract: The combined effects of double substitution of Nb and Al for Fe on the crystallographic structure and magnetic properties of Nd₂Fe_17-x-yNb_xAl_y compounds were investigated by means of X-ray diffraction, neutron diffraction and magnetic measurement. The Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th₂Zn₁₇-type structure with small impurities. The unit cell volumes of Nd₂Fe_17-x-yNb_xAl_y are all found to increase linearly with the increase of Al content and the unit cell volume of Nd₂Fe_16.5-yNb_0.5Al_y is larger than that of Nd₂Fe_17-yAl_y. The neutron diffraction indicates that Nb prefers to occupy 6c sites while Al prefers to occupy 18h sites. The Curie temperature of Nd₂Fe_17-yAl_y first increases and then decreases with the increase of Al content, while the Curie temperature of doubly substituted Nd₂Fe_16.5-yNb_0.5Al_y increase linearly with 0.0≤y≤3.0, and it indicates the synergetic effect on the magnetic property.