An Al2O3P/Al composite was successfully synthesized using a displacement reaction between 80 wt% Al and20 wt% Cu O powders at a heating rate of 5 °C/min. Two different sizes Cu O particles were used, and all the experiments were conducted under an argon atmosphere. To analyze the microstructural evolution during synthesis, the Al–20 wt%Cu O samples were heated to the temperatures selected according to the differential scanning calorimetry curve and then immediately quenched with water. The phase composites and microstructure of the water-quenching samples were investigated using X-ray diffraction, optical microscopy, scanning electron microscopy and energy-dispersive spectrometry.The results indicate that the Cu O particle size has a significant effect on the microstructural evolution of the samples during the heating stage and on the microstructure of synthesized composites. Smaller Cu O particles can decrease the reaction temperature, narrow the reaction temperature range at the different reaction stages during the heating stage and make the size and distribution of in situ Al2O3 particles more uniform. The reaction between Al and Cu O can be complete as the temperature rises to 900 °C. The size of the in situ Al2O3 particles is approximately 5 lm when the size of the Cu O particles is less than 6 lm. This sample has a relatively high Rockwell hardness of 60 HRB. An Al2O3P / Al composite was successfully synthesized using a displacement reaction between 80 wt% Al and 20 wt% Cu O powders at a heating rate of 5 ° C / min. Two different sizes of Cu O particles were used, and all the experiments were conducted under an argon atmosphere. To analyze the microstructural evolution during synthesis, the Al-20 wt% Cu O samples were heated to the temperatures selected according to the differential scanning calorimetry curve and then immediately quenched with water. The phase composites and microstructure of the water- quenching samples were investigated using X-ray diffraction, optical microscopy, scanning electron microscopy and energy-dispersive spectrometry. Results that results that Cu O particle size has a significant effect on the microstructural evolution of the samples during the heating stage and on the microstructure of synthes composites. Smaller Cu O particles can decrease the reaction temperature, narrow the reaction temperature range at the d ifferent reaction stages during the heating stage and make the size and distribution of in situ Al2O3 particles more uniform. The reaction between Al and Cu O can be complete as the temperature rises to 900 ° C. The size of the in situ Al2O3 particles is approximately 5 lm when the size of the Cu O particles is less than 6 lm. This sample has a relatively high Rockwell hardness of 60 HRB.