By comparing the Ostwald ripening behavior of FePt and Fe catalyst films on Al 2 O 3 substrates, we found that the interaction between FePt and Al 2 O 3 is much stronger than that of Fe–Al 2 O 3 . The strong catalyst–substrate interaction guarantees the formation of high area-density, ultrafine and uniform FePt nanoparticles even at elevated temperatures. When used as the catalyst for growing vertically aligned carbon nanotubes (CNTs), the FePt catalyst produced CNTs with good alignment, narrow diameter distribution, and desirable wall-number selectivity. Characterization showed that the vertically aligned single-walled and double-walled CNTs grown from the FePt catalyst possess high purity, good homogeneity, and superior thermal stability. The catalytic mechanism of the FePt binary catalyst is discussed in comparison with that of the pure Fe catalyst.