Abstract: In order to make full use of the cornstalk and prepare the lightweight composites with superior properties, the cornstalk was splited into filamentous structure units according to the analysis of both internal and external surface of it. Then these filamentous structure units were used to fabricate the composite by different formation modes of orientation and non-orientation, and urea formaldehyde resin was used as adhesive. The density of the fabricated composites was 0.3, 0.4, 0.5, 0.6 g/cm3, respectively. The influence of formation structure and density on the physical and mechanical properties of the lightweight composites with cornstalk was explored, including the modulus of rupture (MOR), modulus of elasticity (MOE), internal bond strength (IB), thickness swelling (TS) and water absorption (WA). The results showed that MOR, MOE and IB of the lightweight composites were enhanced by the formation mode of orientation. The MOR∥ and MOE∥ values of OCSB were about 3 times, and the IB value was about 4-5 times as much as HCSB with the same density. The MOR∥, MOE∥ were significantly greater than transverse static bending strength (MOR#x22A5;) and transverse elastic modulus (MOE#x22A5;).When the density was 0.3 g/cm3, MOR∥ was about 3.5 times as much as MOR#x22A5;,MOE∥ was about 5 times as much as MOE#x22A5;;The MOR#x22A5; of OCSB was larger than MOR of HCSB (except 0.3 g/cm3 ), the MOE#x22A5; of OCSB was smaller than MOE of HCSB.The MOR and MOE increased with the increasing of density. When the density was 0.6 g/cm3, the MOR∥of OCSB was 25.24 MPa, the MOE∥ of OCSB reached 4 216 MPa, the physical and mechanical properties can satisfy the civil buildings. In addition, the WA and TS of OCSB were smaller than HCSB.