The ply to ply interlock fabric preform enables to manufacture, by R.T.M. process, thick composite parts that are resistant to delamination and cracking. Numerical simulation of interlock reinforcement forming allows to determine conditions for feasibility of the process and above all to know the position of fibres in the final composite part. For this forming simulation, specific hexahedral finite elements made of seg- ment yarns are proposed. Position of each yarn segment within the element is taken into account. This avoids determination of a homogenized equivalent continuous law that would be very difficult consider- ing the complexity of the weaving. Transverse properties of fabric are taken into account within a hyp- oelastic constitutive law. A set of 3D interlock fabric forming simulations shows the efficiency of the proposed approach.