Abstract: The (Ca, Mg) α′-Sialon/BN composite powder was synthesized by carbothermal reduction nitridation with boron-rich slag as main starting material, of which the composition is (Ca, Mg)xSi12-3xAl3x OxN16-x where x is ranging from 0.3 to 1.8. The effect of different x values on the phase composition and microstructure of synthesized products were investigated by XRD (X-ray diffraction), SEM (scanning electron microscope) and EDX (energy dispersive X-ray). The results showed that the content of α′-Sialon increases with increasing value of x and comes up to the maximum when x=1.4 but decreases when x value exceeding 1.4 with increasing AlN and a certain amount of 15R(SiAl4O2N4) found. The α′-Sialon grains presents long columnar grains when x≥1.4.