MgO-ZrO2 unfired bricks with ZrO2 content up to 8% at the interval of 2% were prepared, using fused magnesia (MgO: 97%) and fused MgO-ZrO2 clinker (ZrO2: 14.33%) as starting materials and phenolic resin as binder. The effects of ZrO2 content on thermal shock resistance (TSR) and other properties such as cold and hot modulus of rupture have been investigated. Residual cold modulus of rupture ratio after heating at 1 000 ℃ and quenching by air blowing was adopted to characterize TSR. Addition of the MgO-ZrO2 clinker improves TSR, attributing to the microcrack toughening effect by thermal expansion mismatch between different phases. When ZrO2 content goes above 4%, the improving effect tends to be moderate. The introduction of MgO-ZrO2 clinker can also improve the HMOR at 1 500℃, while the increased ZrO2 content reduces CMOR of the bricks prefired at 1 600 ℃, due to the thermal expansion mismatch effect. Compromising the overall properties, the optimal ZrO2 content for such magnesia based unfired brick is suggested to be 4%.