By using an appropriate coordinate transform we have calculated the intersubband optical absorption in the single square well under a tilted magnetic field. In this study the dependence of the intersubband transitions on the magnetic field strength and the direction of the magnetic field (tilt angle) is discussed. We show that intersubband optical absorption is sensitive to the tilt angle. This behaviour in the intersubband optical absorption gives a new degree of freedom in regions of interest device applications. (C) 2000 Academic Press.