This work aims at evaluating the transverse and longitudinal coefficients of linear thermal expansion (CLTE) of unidirectional (UD) flax fibres reinforced green epoxy composites. The determination was made by taking length measurements with a calliper and a three-dimensional measuring machine at three discrete temperatures. The longitudinal and transverse CLTE of flax fibre were then estimated from an inverse approach by using two micromechanical models. It was required to use an optimisation program based on the minimisation of the quadratic error. Finally, the internal stresses induced by the thermal expansion were estimated in a symmetric cross-ply laminate ([0/90]s) by a 2D and a 3D analytical model based on classical laminate theory. The thermo-mechanical behaviour of the composite was governed by a linear elastic constitutive law taking into account the expansion strains induced by thermal effects. This was defined firstly without and secondly with taking into account the layer thermal strain in the thickness of the laminate (case 2D and 3D, respectively). The results highlighted the highly anisotropic nature of flax fibres. Indeed, the transverse CLTE was positive and estimated at 75 × 10-6 /°C whereas the longitudinal CLTE was negative and equal to -1.2 × 10-6 /°C. This can be explained by the microstructure and anisotropic properties of wall components of the flax fibre. The results also show that, in the 2D case, externals 0° plies are subject only to a compressive stress of -13.81 MPa in the x-direction and to a tensile stress of 13.81 MPa in the y-direction. Due to the stacking sequence (i.e. symmetric cross-ply laminate), the opposite was expected and finally observed for the internal 90° plies. In the 3D case, the normal stresses increased compared to the 2D case (+17%) while the in-plane shear stress was no longer zero (± 6 MPa).