Abstract We have studied the order–disorder phase transitions of FCC alloy thin films in the (0 0 1) direction using the mean-field method. Surface field is used to describe the surface effects such as surface segregation. All the different types of the phase diagrams of alloy thin films under the surface field are calculated. It is shown that the order–disorder phase transitions of alloy films are much more complex than those of bulk due to the combined effects of surface field and finite size confinement. The main features of phase diagrams are as follows: (1) for the thin films with even number layers, there are some middle-temperature phases due to the anti-phase boundary phenomena induced by the surface field, which leads to more complex phase behavior than those of odd number layer films. (2) For the quasi-two-dimensional thin films with odd number layers, there is an AB ( AB ) n A phase which does not show order–disorder phase transition with the increase of temperature due to the finite size of film. (3) For the thin films with both even and odd number layers, there are some critical points in the phase diagrams.