The effects of 0.02 wt.% Sn addition to Al-4Cu alloy on the phase transformation mechanism from θ′ to θ phase during a long-time heat exposure at 573 K for 100 h and on high temperature strength were studied through microstructure observation and tensile test. It is found that Sn micro-alloying in Al-4Cu alloy completely changes the mechanism of θ′ → θ phase transformation during the long-time heat exposure. In the alloy without Sn addition, rod-like θ phase nucleates and grows on the surface of coarsened disc-like θ? phase. However, with Sn-micro-alloying, θ phase firstly heterogeneously nucleates on β-Sn particles, and then grows into much coarser needle-like phase. Trace addition of Sn significantly increases the yield strength of T5-samples at both room temperature and 573 K, because addition of Sn increases the number density of θ′ particles and reduces its size. But, after the long-time heat exposure, the high temperature (573 K) strength of Al-4Cu alloy is severely degraded because Sn accelerates the phase transformation from θ′ to θ phase.