Abstract: The construction of aliens in American science fictions can be roughly divided into three periods: the aliens in films before 1960s are basically characterized as invaders or enemies, which clearly mirrored cold war mentality; most aliens in films between the 1970s and 1980s aliens were described as kind and friendly, which was the result of the cultural revolution movement in America and the end of the Vietnam war; the aliens in films after 1990s tend to be depicted either as kind or as hostile, which is due to the need of the United States to have new enemies after the cold war.

Key words: films, aliens, science fictions , periods