Abstract: A transfer system for wafer cassettes includes a tray and a rail compatible with the tray. The tray includes a base plate, a through hole, a wall, and a pair of first positioning features. The through hole is disposed on a center of the base plate, in which the through hole has a first direction axis and a second direction axis perpendicular to the first direction axis. The wall extends from a surface of the base plate and surrounding the through hole, in which the wall separates the base plate into an interior region and an exterior region. The pair of first positioning features having a first level height are in the interior region and arranged parallel to the first direction axis.