The effect of thermomechanical treatments (TMT) on the microstructures and properties of Cu-1.5Ni-0.3Si-0.03P-0.05Mg leadframe alloy was investigated. The Cu-base leadframe alloy was received as hot rolled plates with 8 mm thickness. The hot rolled plates were solution treated at 700 degrees C or 800 degrees C for 1 hour, and cold rolled with 40-85% reduction, then followed by aging treatment at 450 degrees C. The leadframe alloy solution treated at 800 degrees C showed larger grain size of 15 mu m comparing with the grain size of 10 mu m in leadframe alloy solution treated at 700 degrees C. The leadframe alloy with smaller grain size of 10 mu m showed higher tensile strength and lower electrical resistivity than that with larger grain size of 15 mu m. The dislocation density increased with increasing reduction ratio of cold rolling from 40% to 85% and resulted in finer Ni2Si precipitates. Tensile strength increased and electrical resistivity decreased with increasing reduction ratio of cold rolling due to the formation of finer Ni2Si precipitates. Two types of thermomechanical treatments were performed to enhance the properties of leadframe alloy. One type of thermomechanical treatment is to refine the grain size through the overaging, cold rolling followed by recrystallization. The recrystallization process improved the tensile strength to 540 MPa and elongation to 15% by reducing the grain size to 5 mu m. The other type of thermomechanical treatment is to refine the precipitate size by two-step aging process. The two-step aging process increased the tensile strength to 640 MPa and reduced the electrical resistivity to 1.475 x 10(-8) Ohm m by reducing the size of Ni2Si precipitates to 4 nm.