An investigation was conducted on the nonlinear behavior of composite sandwich beams made of unidirectional carbon/epoxy facings and PVC foam cores under bending. The carbon/epoxy after an initial linear response exhibits a stiffening nonlinearity in tension and a softening nonlinearity in compression with the longitudinal strength in tension higher than that in compression. This implies that the neutral axis of sandwich beams under bending does not pass through the centroid of the cross section, but is displaced toward the tensile side of the beam. A simple equation was derived and used in conjunction with the stress-strain behavior of the facing material in tension and compression to determine the position of the neutral axis of the beam. Experimental results by moire measurements of the in-plane horizontal displacements of the core material corroborated the analytical predictions. These findings imply higher compressive and smaller tensile stresses in the core. The variation of the normal stresses along the height of the beam is nonlinear and should to be calculated by taking into consideration the nonlinear stress-strain behavior of the core material in tension and compression.