During the roasting process, siderite (FeCO₃) transforms to magnetite (Fe₃O₄) along with producing carbon monoxide (CO), but hematite (Fe₂O₃) needs CO to reduce into magnetite. The process of simultaneously roasting and magnetic separation was developed to treat the low grade siderite and hematite ores without adding any reductant. Effect of mass ratio of S-to-H (siderite ore to hematite ore) on the separation indexes of iron concentrate was discussed. X-ray diffraction was also employed to detect the phase transformations of roasted ores. The technology of simultaneously roasting and magnetic separation is an effective way to treat low grade siderite and hematite ores, and also reduce the emission of CO₂ because no ruductant is added.