Effects of three different modified (i.e., chlorinated, chlorosulfonated, and maleated) polyethylenes (PEs) on the mechanical properties and dimensional stability of recycled PE composites filled with biomass (peat moss) have been investigated at room temperature and after exposure to boiling water for 24 h. From the experimental results, it is suggested that differently modified PEs play an important role in improving the physicomechanical properties of the peat moss-filled recycled PE composites, even after exposure to boiling water. However, maleated PE is by far the best coupling agent as far as mechanical properties and dimensional stability of the peat moss-filled recycled PE is concerned. The effects of concentrations of peat moss and coupling agent (i.e., chlorinated PE) on the properties of the composites have been statistically analyzed based on an experimental design and on an empirical quadratic model.