A calculation of the exchange interaction constant between the R moment and the T moment in rare-earth (R) transition-metal (T) compounds of the type R2Fe17 has been discussed using molecular field theory combining with the results of neutron diffraction or X-ray diffraction. The calculated values for R2Fe17-xAlx (R=Tb,Dy,Ho,Er and Gd,x=7,8) have been given and they are in agreement with the experimental results .