The precipitation effect of Al-6%Si-0.4%Mg-0.9%Cu-(Ti) alloy (in wt.%) after various heat treatments was studied using a laser flash device (LFA) and differential scanning calorimetry (DSC). Solid solution treatment was performed at 535 °C for 6 h, followed by water cooling, and samples were artificially aged in air at 180 °C and 220 °C for 5 h. The titanium-free alloy Al-6%Si-0.4%Mg-0.9%Cu showed higher thermal diffusivity than did the Al-6%Si-0.4%Mg-0.9%Cu-0.2%Ti alloy over the entire temperature range. In the temperature ranges below 200 °C and above 300 oC, the value of thermal diffusivity decreased with increasing temperature. As the sample temperature increased between 200 °C and 400 °C, phase precipitation occurred. From the results of DSC analysis, the temperature dependence of the change in thermal diffusivity in the temperature range between 200 °C and 400 °C was strongly influenced by the precipitation of θ'-Al2Cu, β'-Mg2Si, and Si phases. The most important factor in the temperature dependence of thermal diffusivity was Si precipitation.