Abstract: In order to find the best breakage mechanism of RO phase liberation in steel slag, four kinds of steel slag powder with different particle size distribution have been comminuted respectively by high-pressure grinding rolls (roller press), vertical roller mill (vertical mill) and ball mill. Each of them was actually sieved into 7 size fractions then made into polished section. The proportion of RO phase grain in the five liberation classes and the phase surface parameters were manually counted. The liberation properties were characterized by the two index parameters of liberation degree and the amount of 75%~100% liberated, while the amount of liberation by detachment was described by phase specific interfacial area and free surface percentage. As the RO phase in steel slag is easy to be released from the rock matrix, the amount of liberation by detachment of four steel slag powders is large. The priority sequence of comminution equipment in terms of the amount of liberation by detachment is roller press, vertical mill, ball mill. The finer the steel slag powder made by the same equipment, the better the amount of liberation by detachment. Liberated minerals could be divided into two regions by the average dissemination size of RO phase. The first region is a stability zone for liberation properties, in which the degree of liberation is independent of particle size and device. The second region is a decline zone, in which the liberation degree significantly decreases with increasing the particle size.