During the past two decades the use of lignocellouse fibers in reinforcing composites has attracted much research activities. In the present work, date palm fiber was used for production of composites compatible withliving environment. The fibers were pre-treated chemically to remove impurities. In order to verify and compare the effectiveness of the pretreatment methods, lignin, ash, moisture adsorption, diameter and tensile strength of the raw and treated fibers were considered in all determinations. Some chemical contents of the treated fibers were also estimated by FTIR method. The heating characteristics of the fibers were evaluated using simultaneous thermal analysis (SAT) technique. The treated fibers were mixed with HDPE by mass proportions of 10, 20 and 30% of the fibers in twotypes of fine and coarse sizes. Composite flower pots were prepared from the formulations by extrusion process. The mechanical properties of the composites including tensile strength, modulus of elasticity, strain, and impact strength were measured by standard ASTM methods. Statistical analysis of the data revealed that the treated fibers had smaller diameters containing lower levels of lignin and ash though having significantly higher tensile strength, heat resistance and moisture adsorption. The results also indicated that by increase in fibre size the tensile strength, modulus of elasticity and moisture adsorption of the composites are increased and their strain and impact resistance are decreased. The composites prepared using fine fibers showed higher tensile strength, modulus of elasticity and impact strength but their moisture adsorption and elongation were significantly lower.