We present theoretical study of the third type thermomechanical effect in twist aligned nematic liquid crystal cell. Euler- Lagrange-Rayleigh variation method is applied together with the heat-transfer equation to analyze the laser-induced nonlinearity and director reorientation due to the third type thermomechanical effect. Director reorientation due to a transversal 2-dimensional temperature gradient is considered. Director equation is solved numerically and phase shift distribution is presented. It was explained that phase shift is zero in some areas because temperature gradient x and y components compete with each other to reorient the director in opposite directions. It was shown that maximum phase shift is at the same range as that for the Giant Optical Nonlinearity (GON).© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.