In this work, the mechanical and morphological properties of high density polyethylene (HDPE)/flax fiber composites are reported. In particular, the effect of adding a rubber phase (SBR) in terms of recycled ground tire rubber (GTR) is determined. Samples with different concentrations of flax fibers and rubber powders were prepared. Also, the addition of a coupling agent (SEBS) on both matrix-fiber and matrix-rubber surface adhesion and composite morphology is investigated. The results show an improvement in both matrix-rubber and matrix-fiber surface adhesion after incorporation of the coupling agent leading to improved tensile and flexural properties. Also, impact strength results showed a toughening effect of the HDPE matrix by flax and SBR addition.