Environment-friendly, fully biodegradable, ‘green’ composites based on plant based fibers and resins are increasingly being developed for various applications as replacements for non-degradable materials derived from petroleum that are currently being used. Unlike petroleum, plant based proteins, starches and fibers are yearly renewable. In addition, these green composites may be easily composted after their life, completing nature's carbon cycle. In this study, soy flour (SF) was modified by cross-linking it with glutaraldehyde (GA). The cross-linked soy flour (CSF) polymer was characterized for its tensile and thermal properties. The effect of glycerol on the mechanical properties of the soy flour was characterized and optimized. CSF polymer showed improved tensile properties and thermal stability, compared to unmodified SF resin, for use as a resin to fabricate composites. Unidirectional green composites using flax yarn and CSF resin were fabricated and characterized for their tensile and flexural properties. The composite specimens exhibited fracture stress and Young's modulus of 259.5 MPa and 3.71 GPa, respectively, and flexural strength of 174 MPa, in the longitudinal direction. These properties seem to be sufficient for considering these green composites for indoor structural applications.