Abstarct: In present work, buckling behavior of functionally graded carbon nanotube-reinforced composite (FG-CNTRC) beams in hygrothermal environments baxsed on different theories of shear deformation is investigated. The material properties are significantly simulated in line with the beam thickness and the boundary conditions of the beam are considered simply support. The properties of these materials are dependent on temperature and humidity. The distributions of temperature and humidity along the thickness line are uniform. In order to extract the equilibrium equation from shear deformation theories, the first and higher order shear deformation theories are used. The governing equations considering of the temperature and moisture are extracted using the energy method. The results of the research are verified with other references. The results show that the carbon nanotube- reinfoced composite beam is much more resistant to bucklimg than graphite/epoxy composite beam. The effects of varions shear deformation theories on the temperature, moisture and amplitude of the buckling of FG-CNTRC is investigated. The FGX and FGO distributions of the FG-CNTRC beams have the highest and lowest buckling temperature, respectively.