In this work, the effect of magnetite with different particle sizes on the flotation performance of both coarse and fine hematite particles were investigated by using sodium oleate as a collector. The results showed that the magnetite particles with different particle sizes showed a negative effect on hematite (-106+45 μm) recovery, but the addition of magnetite with the same particle sizes as hematite during the direct flotation of -45 μm hematite was beneficial to improve the recovery of micro-fine hematite and the Fe grade of concentrate. The finer the magnetite particle was, the more obvious the agglomeration effect of hematite was. Therefore, the beneficial effect could be achieved by adjusting the particle sizes of particles. Moreover, sodium oleate was beneficial to promote the agglomeration of micro-fine magnetite and hematite. The results from the microscopic analysis, laser particle size analysis, and EDLVO calculation proved that there was an effective aggregation between fine magnetite and fine hematite particles, which increased the apparent size of hematite particles and the probability of the mineral particles adhering to bubbles, thus improving the hematite recovery.