In this work, high density polyethylene (HDPE) was compounded with chemically treated coir fiber using a heated two roll mill. Three chemical treatments denoted silane, sodium hydroxide (NaOH) and dodecane bromide (C12) were selected to improve the interface adhesion between fibers and polyethylene matrix. The mechanical properties of these composites were evaluated and compared against those of neat polymer and untreated fibers composites. A fixed fiber loading of 20wt.% was used in all composites. A chemical analysis using Fourier Transform Infrared spectroscopy (FTIR) was performed to see the extent of chemical modification of the fibers. Results have shown that tensile and torsional modulus exhibited a significant increase when compared to the neat HDPE. A notable increase in the Young’s modulus was observed when C12 and silane were used, corresponding to 120% and 70%, respectively. The composites’ ductility was also evaluated by the plastic stored energy and showed a noted increase when C12 and silane were used, an increase of 55% with C12 treatment and 23% when silane treatment was used. Such results promise many applications for composite materials.