Abstract: Coal, fly ash and slag are common cement admixtures and concrete additives of which the granularity is one of the impact factor on powder properties.In this paper, slag powder is prepared in JFC-5 jet mill and the character and morphology of the powder are investigated.The results indicate that the specific surface of the powder reaches a stable value as the milling conducting, the particle size is centrally distributed and when rotate speed of the mill separator is too high, the slag powder present reunite phenomenon.