Date palm leaves and wood are common wastes in the middle east. These natural fibers can be used as additives for polymers to develop new desired properties; they also have important economic and environmental advantages. The major problem in the addition of the natural fiber to the polymer matrix is the poor interface between the hydrophilic lignocellulosic fiber and the hydrophobic polymer matrix. Compatibilizers are usually used to overcome this problem. This work is focusing on the effect of the date palm filler size and origin (leaf or trunk) from female (Ikhlas) on the high density polyethylene matrix. The following interfaces were used to compatibilize the matrix and the fillers: Polyethylene-co-methacrylic acid (PEMA), Polyethylene-graft-maleic anhydride (PEgMA) and Polyethylene-co-methacrylic acid zinc salt (PEMA-Zn) with 1, 2 and 3 wt.%. Morphologies of composites were carried out by Scanning Electron Microscope (SEM). The mechanical and thermal properties of different samples were evaluated by tensile and TGA apparatus respectively. Samples based on 1 wt.% PEMA shown the highest Young’s modulus This behavior is may due to the increase in the interfacial adhesion and dispersion of the palm fillers within the polymer matrix, and confirmed by SEM micrographs. Thermal stability as limited by the compatibilizers due to the low thermal stability of the natural fillers itself.