The influence of isothermal annealing on mechanical properties of the Cu-clad Al (CA) wire was experimentally examined to better understand the annealing mechanisms that occur in the CA wire. In the experiment, the CA wire was prepared by drawing to decrease the diameter d from 10 mm to 2.9, 1.5 and 0.5 mm. The CA wires with these three different diameters were isothermally annealed at temperatures of 423–603 K (150–330°C) for various periods between 10.8 ks and 3456 ks (3 and 960 h). At room temperature, tensile tests were performed on the CA wire using an Instron type testing machine, and hardness tests were made on the Cu layer and the Al core of the CA wire utilizing a micro Vickers hardness testing machine. For the CA wire without annealing, the ultimate tensile strength su is 231, 215 and 198 MPa for d = 0.5, 1.5 and 2.9 mm, respectively, and the elongation eu is 0.4, 0.4 and 1.2% for d = 0.5, 1.5 and 2.9 mm, respectively. Due to isothermal annealing, recovery and recrystallization take place in both the Cu layer and the Al core of the CA wire, and an intermetallic layer consisting of various Cu–Al compounds forms at the Cu/Al interface. The intermetallic layer promotes formation of cavity at the Cu/Al interface during the tensile test. As the thickness l of the intermetallic layer reaches to the critical thickness lm, su attains to the minimum value, and eu takes the maximum value. Here, lm = 1–2 µm for d = 0.5–2.9 mm, respectively. For l > lm, however, su is insensitive to l, but eu decreases with increasing thickness l. Thus, the appropriate combination of the annealing temperature and time is essentially important to realize the optimal mechanical properties of the CA wire.