The melt spun ribbons of nanocomposite Nd9Fe85.5-xCoxNb1B4.5(x＝0-16) have been prepared by rapid quenching and post heat treatment. The best magnetic properties obtained in Nd9Fe84.5Nb1B4.5 ribbons are as follows:(BH)max＝156kJ/m3,Jr＝1.11T and μ0HcJ＝0.68T.It is shown that the melt spun ribbons consist of both 2∶14∶1 compound and α-Fe(Co,Nb) phases with mean grain sizes smaller than 30nm. The effect of exchange coupling between the nanostructure grains of Nd2(Fe,Co,Nb)14B and α-Fe(Co,Nb) are observed clearly. The Curie temperature enhancement is about 10K and the largest reduced remanence achieved 0.776.It is also indicated that there is a close relation between the reduced remanence and the mean grain size of α-Fe phase. The smaller the mean grain size of α-Fe phase, the greater the remanence enhancement. The Curie temperature of the melt spun ribbons increases quickly with a rate of 11.3K/at%. The coercivity, remanence and maximum energy product decrease monotonically with increasing Co content for the Nd9Fe85.5-xCoxNb1B4.5 ribbons.