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In situ formed low density O’-sialon-based multiphase ceramics were prepared by liquid-phase sintering method at 1400℃ with Si_3N_4, SiO_2 and Al_2O_3 as raw materials. Crystalline phases were identified by X-ray diffraction (XRD). The quantitative phase analysis was finished by matrix-flushing method and the substitution parameter x value of O’-sialon was estimated. The effects of sintering additives on the phase composition of the material were studied. The results show that, when using Y_2O_3 alone, Al_6Si_2O_(13) phase can be formed in the material, but when using Y_2O_3 and MgO, MgAl_2O_4 phase can be preferentially formed and the Al_6Si_2O_(13) is not observed. The mechanical properties of the material were measured and the relationships between microstructure and mechanical properties were discussed. The sample with Y_2O_3 and MgO sintering additives, using fused quartz alone as SiO_2 source, displays a combination of high bending strength (163 Mpa) and good fracture toughness (3.11 Mpa·m~(1/2)). Bending strength and fracture toughness of the samples increase with the increase of the content and aspect ratio of elongated grains and decrease with the increase of the porosity.