Pyrrhotite solid solution, subsequent to crystallization from sulfide melt, will suffer strong annealing during slow cooling. Annealing is represented by both of the two processes, exsolution of the solid solution and grain coarsening of the exsolution products including lamellar, monoclinic pyrrhotite and flame-like pentlandite. Although high-temperature exsolution mechanism has been recognized as a major mechanism for the formation of granular pentlandite included inside pyrrhotite grains or occurring along grain boundaries between pyrrhotite and other minerals, coarsening of flame-like pentlandite is considered by the present authors as another possible mechanism. Exsolution of monoclinic pyrrhotite and coarsening of the exsolution lamellae will modify the magnetic property of pyrrhotite in ores, whereas coarsening of exsolved pentlandite will increase the size of this nickel-bearing mineral. As a result, annealing will have significant effect on the processing properties of ores.