Abstract: The vibration and buckling behaviors of porous functionally graded material(FGM) beams under the action of axial mechanical load in hygrothermal environment were investigated by an extension of a n-th order generalized beam theory (GBT). The material properties were temperature-dependent and described by modified Voigt mixture rule with porosity. The free vibration and buckling equations of the system were obtained by using the macro-micro analytical approach and Hamilton principle, in which three types of hygro-thermal distribution through the thickness of a beam were assumed. Applying the Navier solution method, the solutions for the free vibration and buckling responses of FGM simply supported beams were presented. The availability and accuracy for the GBT were tested throughout the numerical results and herein the satisfactory values to n was proposed, which can also refine beam theories. The effects of three types of hygro-thermal distribution, coupling hygro-thermal-mechanical loads, porosity, material graded index and length-to-thickness ratio on the vibration and buckling behaviors of a FGM beam were discussed. The results show that frequency and buckling load of the structure decrease as both temperature and moisture rise, and different types of hygro-thermal distribution will lead to distinct effects on it. As the porosity of the material increases, the structural unitary stiffness will be weakened, while frequency and stability of the structure in hygro-thermal environment will increase. Hygro-thermal rise has a little effect for short and thick FGM porous beams but remarkable effect for long and thin ones on the frequency and stability.