The coercivity of mechanically milled nanocomposites of Nd4.25Fe77.75B18 (H-c = 175 kA/m) was improved by similar to 60% with the substitution of heavy rare-earths and Co, Nd3.25Tb1Fe71.75Co5SiB18 (H-c = 279 kA/m). The ternary composition was systematically varied to improve the magnetic properties and understand the coercivity mechanism. The rare-earth components were varied to increase the magnetocrystalline anisotropy of the hard phase. The highest coercivity attained for a rare-earth only addition was H-c=250 kA/m (Pr3.25Tb1Fe72.75Co5B18). Additions of Si, Ti, Cu0.5Nb0.5, Cr and Fe improved the magnetic properties further, Mr = 1.11 T, H-c = 264 kA/m and (BH)(max) = 75 kJ/m(3). These data with Mossbauer spectra provide an insight into why these compositions have higher coercivities over the ternary alloy but lower coercivities compared with similar melt-spun samples.