A method for preparing a microcrystalline alloy for manufacturing a R-Fe-B rare earth sintered magnet comprising Nd 2 Fe 14 B crystal phase as major phase, wherein R is an element or a combination of two or more elements selected from rare earth elements inclusive of Sc and Y and contains Nd and/or Pr, said method comprising: step (a) of strip casting an alloy having the composition R 1 a T b M c A d wherein R 1 is an element or a combination of two or more elements selected from rare earth elements inclusive of Sc and Y and contains Nd and/or Pr, T is Fe or Fe and Co, M is a combination of two or more elements selected from the group consisting of Al, Cu, Zn, In, P, S, Ti, Si, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, Ta, and W and contains Al and Cu, A is B (boron) or B and C (carbon), "a" to "d" indicative of atomic percent in the alloy are in the range: 12.5 ≤ a ≤ 18, 0.2 ≤ c ≤ 10, 5 < d ≤ 10, and the balance of b, and consisting essentially of crystal grains of Nd 2 Fe 14 B crystal phase and precipitated grains of R 1 -rich phase, the grains of R 1 -rich phase being precipitated in such a distribution that the average distance between precipitated grains is up to 20 μm, thereby obtaining a strip cast alloy, characterised in that the method further comprises step (b) of HDDR treatment of heating the strip cast alloy in hydrogen atmosphere at 700 to 1,000°C to induce disproportionation reaction to disproportionate the Nd 2 Fe 14 B crystal phase into R 1 hydride, Fe, and Fe 2 B, then heating the alloy under a reduced hydrogen partial pressure at 700 to 1,000°C to recombine them into Nd 2 Fe 14 B crystal phase, thereby forming submicron crystal grains having an average grain size of 0.1 to 1 μm, thereby obtaining an HDDR-treated alloy, and step (c) of diffusion treatment of heating the HDDR-treated alloy in vacuum or in an inert gas atmosphere at a temperature of 600 to 1,000°C for a time of 1 to 50 hours, thereby preparing a microcrystalline alloy having a structural morphology consisting of Nd 2 Fe 14 B major phase crystal grains having an average grain size of 0.1 to 1 μm and R 1 -rich grain boundary phase surrounding the crystal grains across an average width of 2 to 10 nm. step (a) of strip casting an alloy having the composition R 1 a T b M c A d wherein R 1 is an element or a combination of two or more elements selected from rare earth elements inclusive of Sc and Y and contains Nd and/or Pr, T is Fe or Fe and Co, M is a combination of two or more elements selected from the group consisting of Al, Cu, Zn, In, P, S, Ti, Si, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, Ta, and W and contains Al and Cu, A is B (boron) or B and C (carbon), "a" to "d" indicative of atomic percent in the alloy are in the range: 12.5 ≤ a ≤ 18, 0.2 ≤ c ≤ 10, 5 < d ≤ 10, and the balance of b, and consisting essentially of crystal grains of Nd 2 Fe 14 B crystal phase and precipitated grains of R 1 -rich phase, the grains of R 1 -rich phase being precipitated in such a distribution that the average distance between precipitated grains is up to 20 μm, thereby obtaining a strip cast alloy, characterised in that the method further comprises step (b) of HDDR treatment of heating the strip cast alloy in hydrogen atmosphere at 700 to 1,000°C to induce disproportionation reaction to disproportionate the Nd 2 Fe 14 B crystal phase into R 1 hydride, Fe, and Fe 2 B, then heating the alloy under a reduced hydrogen partial pressure at 700 to 1,000°C to recombine them into Nd 2 Fe 14 B crystal phase, thereby forming submicron crystal grains having an average grain size of 0.1 to 1 μm, thereby obtaining an HDDR-treated alloy, and step (c) of diffusion treatment of heating the HDDR-treated alloy in vacuum or in an inert gas atmosphere at a temperature of 600 to 1,000°C for a time of 1 to 50 hours, thereby preparing a microcrystalline alloy having a structural morphology consisting of Nd 2 Fe 14 B major phase crystal grains having an average grain size of 0.1 to 1 μm and R 1 -rich grain boundary phase surrounding the crystal grains across an average width of 2 to 10 nm.