Isotropic magnets were prepared by hot-pressing/die-upset process with Nd9Fe85.5-xCoxB5.5(x=0, 1, 3, 5) melt-spun ribbons as starting material at the wheel surface speed V=35 m/s. The microstructure analysis indicates that Co-doped alloy melt-spun ribbons consists of a large amount of amorphous phase and a little amount of α-(Fe, Co) and Nd2(Fe,Co)14B, and Nd3(Fe,Co)62B14 metastable phase exists in the melt-spun ribbons with x=5. The metastable phase will decomposes into α-(Fe,Co) and Nd2(Fe,Co)14B mixture during the following hot-pressing/die-upset process. The average grain sizes of both soft and hard magnetic phases decrease respectively from 61 and 168 nm to 24 and 50 nm, with the Co content increasing from 0 at% to 1at%. The magnetic property research show that corresponding to the change of grain sizes, magnetostatic coupling interaction is dominant for Co-free alloy, while exchange coupling interaction is dominant for Co-doped alloys. The effect of exchange-coupling becomes weaker with more Co-doping, resulting in the monotonous increasing of the intrinsic coercivity from 151 kA/m for Co-free magnet to 218 kA/m for 5 at% Co magnet.