This work aimed to develop a new biocomposite material that could be used as thermal insulation in buildings. The goal was to investigate experimentally the effect of date palm fibers DPF (Phoenix dactylifera L. from Biskra oasis in Algeria) on thermal conductivity, water absorption and mechanical properties of gypsum based materials. Biocomposites containing various DPF filler contents and two different sizes of DPF were prepared. The results showed that the thermal conductivity of the gypsum based materials decreases with increasing the DPF concentration. We have shown that DPF loading may induce a high effect on the mechanical and thermal properties of the composites than the size fiber effect. The compressive and flexural strength of the biocomposites can be improved by adding the adequate fibers contents. This new kind of biocomposites exhibit good thermal and mechanical performances which allow it to be applied as thermal insulation materials. (C) 2013 Elsevier B.V. All rights reserved.