Thermomechanical nonlinearity caused by transversal temperature gradient was experimentally investigated. Temperature dependence of this effect is investigated in twist aligned nematic liquid crystal cell (with thickness of 100μm). It is shown that the effect is orientational and non-diffractional. It is also shown that the orientational optical nonlinearity of revealed thermomechanical effect may be as strong as well-known giant optical nonlinearity. Besides, at normal incidence of the laser the giant optical nonlinearity does not exist, but thermomechanical effect exists at any angle of incidence of the laser and the nonlinearity of thermomechanical effect remains in the same order.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.