Abstract The use of short palm tree lignocellulosic fibers as a reinforcing phase in polyester matrix has been reported. In order to improve interfacial adhesion, the esterification of the lignocellulosic filler in alkaline medium was performed using maleic anhydride. The dynamic dielectrical analysis of short palm tree lignocellulosic fibres reinforced polyester composites was carried out. Measurements were performed in the frequency range 1–104 Hz. Two relaxations processes were identified in the case of polyester matrix and non treated fibre composite, which are the α relaxation associated with glass–rubbery transition of the polymer and the relaxation process associated with conductivity occurring as a result of the carriers charges diffusion for high temperature above glass transition and low frequencies. For the treated fiber composite with anhydride, a new relaxation process due to the molecular motion of amorphous celluloses was identified.