This paper presents an experimental study on the behavior of sandwich beams made of green materials for both core and skin components. A unidirectional flax fabric and a partial bio-based epoxy were used to make fiber-reinforced polymer (FRP) skins and three flute varieties of corrugated cardboards (known as B, C, and BC flutes) with the bulk densities of 170, 127, and 138 kg/m3 were used for the core, respectively. A total of 30 small-scale sandwich beam specimens were manufactured across six unique beam varieties with dimensions of 50 mm in width, 25 mm in depth, and 200 and 350 mm in length (150 mm and 300 mm spans) and tested under four-point bending up to failure. Two failure modes of transverse indentation for the short specimens and longitudinal crushing of the core and skin for the long specimens were observed. The load-deflection, load-strain, and moment-curvature behaviors were analyzed to evaluate the strength and stiffness of the sandwich beam specimens. C flute with the lowest bulk density and the highest availability in the market amongst all the three flutes exhibited the highest strength and stiffness for sandwich applications. Overall, the corrugated cardboard cores combining with the flax FRP skins may be considered as a viable, green option for the fabrication of large-scale structural sandwich panels for building applications. •Sandwich beams made of green materials were tested under bending.•Corrugated cardboards and bio-based flax composite skins were used.•Strength and stiffness of the sandwich beams were evaluated.•An analytical model considering nonlinearity of skins was developed.•It was concluded that corrugated cardboards can be considered a viable core material.