Aging behavior of Cu-3 at.% Ti-4 at.% Al alloy at 723 K has been examined from mechanical, electrical, and microstructural points of view. Compared with binary Cu-3 at.% Ti alloy, the electrical conductivity improved six times to about 6%IACS (international annealed copper standard); whereas the peak hardness decreased from 280 to 180 Hv. The major strengthening phase is the tetragonal alpha-Cu4Ti, which forms not via spinodal decomposition but based on the nucleation and growth mechanism. The precipitates grow in the c direction of the tetragonal phase, which lies along one of the < 100 > axes of the matrix fcc Cu phase. This growth mode minimizes the strain energy arising from the lattice mismatch of about 2% between the matrix and precipitate; and results in a square rod shape, which reaches about 50 nm in length after 100 h anneal. Another precipitating phase is AlCu2Ti (D0(3), Strukturbericht notation), with the major habit plane close to {110} of the fcc Cu matrix. The orientation relationship was not definitely determined, but it was found that the angle between the 100 and 110 poles of the matrix and precipitates, respectively, is about 5 degrees, while the angle between the two 001 axes being about 7 degrees. It was suggested that the formation of this ternary phase reduced the solute Ti concentration, leading to the decrease in the resistivity.