This article studied the effect of phase transformation on the thermal expansion property in Al/ZrW2O8 composites. The Al/ZrW2O8 composites of low-thermal expansion were fabricated by a squeeze casting method. The coefficient of thermal expansion (CTE) of as-made composites was discovered sharply increased at around 130 A degrees C. The X-ray diffraction (XRD) spectra showed the existence of high-pressure gamma-phase in the as-made composites. This high-pressure gamma-phase was considered to be induced by the compressive residual stress originated from the thermal mismatch between Al matrix and ZrW2O8 particles. The in situ high-temperature XRD and the differential scanning calorimetry technique were used to study this thermally expanded abruption phenomenon. It was found that the phase transformation from high-pressure gamma-phase to the low-pressure phases (alpha/beta phase) in the composites should be responsible for fluctuation in the CTE of composites. Furthermore, using a proper heat treatment to eliminate the high-pressure phase in the composite, the Al/ZrW2O8 composites of low and uniform CTE (from 20 to 200 A degrees C) could be achieved. And when temperature increased again, the thermal mismatch stresses between the metal matrix and ceramic particles in the composite were not large enough to re-induce the alpha-gamma transformation.