The effect of grain size on the magnetic properties of a rapidly solidified low boron Fe-Nd-B alloy, both in directly quenched and in the overquenched and annealed conditions, has been studied. The magnetic properties in the as-quenched crystalline state and changes occurring on annealing of the overquenched ribbons are discussed in relation to the microstructure. High values of the remanent magnetization, above 0.9 T (9 kG), in combination with coercivities of 1100 kA m-1 (13.7 kOe), have been observed for the directly quenched, isotropic single-phase ribbons, for certain combinations of process conditions. The mean-grain size for these ribbons measured on the roll contact side was found in some cases to be <20 nm, representing an essentially nanocrystalline structure. The enhanced remanence is considered to result from intergranular magnetic exchange interactions between fine adjacent grains. The overquenched ribbons were aged at temperatures between 500 and 800°C for various times and these exhibited grain sizes in the range of 40-55 nm. Their magnetic properties depend on the annealing temperature and are generally lower than for the directly quenched ribbons though Br is still slightly enhanced at 0.85 T (8.5 kG). Apart from the effect of larger grains, the lower magnetic properties of the overquenched and annealed ribbons result from a slightly different phase constitution. Indirect evidence of this is provided by initial magnetization and coercivity measurements, using a VSM, which indicate that, on devitrification of the overquenched amorphous phase, a small proportion of soft magnetic phase was always present but which was not observed in the optimal directly quenched samples. Details of the phase transformations are discussed.