A method for preparing an R-Fe-B base sintered magnet of a composition consisting essentially of 12 to 17 at% of R which is at least two of yttrium and rare earth elements and essentially contains Nd and Pr, 0.1 to 3 at% of M 1 which is at least one element selected from the group consisting of Si, Al, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi, 0.05 to 0.5 at% of M 2 which is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, 4.8+2×m to 5.9+2×m at% of B wherein m stands for atomic concentration of M 2, up to 10 at% of Co, up to 0.5 at% of carbon, up to 1.5 at% of oxygen, up to 0.5 at% of nitrogen, and the balance of Fe, containing R 2 (Fe,(Co)) 14 B intermetallic compound as a main phase, and having a coercivity of at least 800kA/m at room temperature, wherein the magnet contains an M 2 boride phase at grain boundary triple junctions, but not including R 1.1 Fe 4 B 4 compound phase, has a core/shell structure that the main phase is covered with grain boundary phase comprising an amorphous and/or sub-10 nm nanocrystalline R-Fe(Co)-M 1 phase consisting essentially of 25 to 35 at% of R, 2 to 8 at% of M 1, up to 8 at% of Co, and the balance of Fe, or the R-Fe(Co)-M 1 phase and a crystalline or a sub-10 nm nano-crystalline and amorphous R-M 1 phase having at least 50 at% of R, wherein the R-Fe(Co)-M 1 phase exists outside of and surrounding the main phase, and wherein a surface area coverage of the R-Fe(Co)-M 1 phase on the main phase is at least 50%, and the width of the intergranular grain boundary phase is at least 10 nm and at least 50 nm on the average, the method consisting of the steps of: shaping an alloy powder into a green compact, the alloy powder being obtained by finely pulverizing an alloy consisting essentially of 12 to 17 at% of R which is at least two of yttrium and rare earth elements and essentially contains Nd and Pr, 0.1 to 3 at% of M 1 which is at least one element selected from the group consisting of Si, Al, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi, 0.05 to 0.5 at% of M 2 which is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, 4.8+2×m to 5.9+2×m at% of B wherein m stands for atomic concentration of M 2, up to 10 at% of Co, and the balance of Fe, sintering the green compact at a temperature of 1,000 to 1,150°C, cooling the sintered compact to a temperature of 400°C or below, post-sintering heat treatment including heating the sintered compact at a temperature in the range of 700 to 1,100°C which temperature is exceeding peritectic temperature of R-Fe(Co)-M 1 phase, and cooling down to a temperature of 400°C or below at a rate of 5 to 100°C/min, and aging treatment including exposing the sintered compact at a temperature in the range of 400 to 600°C which temperature is lower than the peritectic temperature of R-Fe(Co)-M 1 phase so as to form the R-Fe(Co)-M 1 phase at a grain boundary, and cooling down to a temperature of 200°C or below. shaping an alloy powder into a green compact, the alloy powder being obtained by finely pulverizing an alloy consisting essentially of 12 to 17 at% of R which is at least two of yttrium and rare earth elements and essentially contains Nd and Pr, 0.1 to 3 at% of M 1 which is at least one element selected from the group consisting of Si, Al, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi, 0.05 to 0.5 at% of M 2 which is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, 4.8+2×m to 5.9+2×m at% of B wherein m stands for atomic concentration of M 2, up to 10 at% of Co, and the balance of Fe, sintering the green compact at a temperature of 1,000 to 1,150°C, cooling the sintered compact to a temperature of 400°C or below, post-sintering heat treatment including heating the sintered compact at a temperature in the range of 700 to 1,100°C which temperature is exceeding peritectic temperature of R-Fe(Co)-M 1 phase, and cooling down to a temperature of 400°C or below at a rate of 5 to 100°C/min, and aging treatment including exposing the sintered compact at a temperature in the range of 400 to 600°C which temperature is lower than the peritectic temperature of R-Fe(Co)-M 1 phase so as to form the R-Fe(Co)-M 1 phase at a grain boundary, and cooling down to a temperature of 200°C or below.