Herein, we report a model investigation of the adhesion properties between major polysaccharides present in flax fibre and polylactic acid (PLA) polymer matrix, and a correlation between these interactions and the final mechanical properties of the fibre and the composite. The raw flax fibre was first submitted to two separate treatments, an enzymatic and an alkali one, which are known to indirectly promote the fibre-matrix interface adherence. Morphology, mechanical and adherence properties of the treated fibres were measured from nanometer to macroscopic scale. In a second time, we adapt the colloidal force microscopy (CFM) to directly probe the interaction forces between PLA and different polysaccharides in the flax fibre. Our results underline the important interactions of cellulose and PLA matrix with the hydrated polymers, such as hemicelluloses and pectins that plays crucial role in maintaining fibres intrinsic strength, as well as interfacial properties respectively. In parallel, adhesion force mapping of real raw and treated flax fibre's surface by AFM force-volume technique give complementary results that underline an important adhesion with the hydrated polysaccharides.