Large number of studies has been made about renewable bio-based (e.g. flax, hemp reinforced composites) composites during the last years. It has been showed that mechanical properties of bio-based composites compares well with glass fiber composites, especially if the properties are normalized with density. However, there are some drawbacks with respect to utilization of natural fiber properties to the extent that composites based on these reinforcements also can compete with synthetic materials, such as limited fiber length, sensitivity to the moisture, temperature etc. But probably one of the major drawbacks of natural fibers is variability of properties due to growth, processing conditions etc. Therefore, another type of reinforcement with high cellulose content is of interest – regenerated cellulose fibers (RCF). These fibers are manmade, but unlike materials with fossil origin, they are made out of the natural polymer directly. RCF is continuous and therefore it is easy to arrange them into fabrics with stable orientation and geometry. But RCF fiber and also bio-based resin performance is highly non-linear with presence of very significant viscoelastic and viscoplastic component.As basis material model developed by Schapery [1,2] was used in order to analyse time dependent behaviour of these materials. Model has been further modified to account for microdamage [3] In order to obtain all parameters needed for this model large number of experiments needs to be carried out. The numerous combinations of matrix and fibers are possible. Therefore it would be more convenient if material model is based on performance of constituents and test on composites should be performed only for verification of the model.The main objective of this investigation is to predict mechanical behaviour of these composites and their constituents by generalizing existing models to capture their time-dependent behaviour. In order to identify and quantify parameters needed for the modelling, extensive damage tolerance tests as well as creep experiments are carried out. REFERENCES1. Lou Y.C. and Schapery R.A., “Viscoelastic Characterization of a Nonlinear Fibre-reinforced Plastic,” Journal of Composite Materials, Vol. 5, pp. 208-234, 1971. 2. Schapery R.A., “Viscoelastic and Viscoplastic Constitutive Equations Based on Thermodynamics,” Mechanics of Time-Dependent Materials, Vol. 1, pp. 209-240, 1997. 3. Marklund E., Eitzenberg J. and Varna J. “Nonlinear viscoelastic viscoplastic material model including stiffness degradation for hemp/lignin composites,” Composite Science and Technology, Vol. 68, pp. 2156-2162, 2008.