The availability of natural fibers in Egypt such as bagasse, rice straw, corn husk, etc attracted the researcher interests to reutilize this agricultural biomass for the development of natural fiber reinforced composites for diversity valuable applications. Recently the utilization of bio-fillers in thermoplastics composites has increased. Polystyrene is a thermoplastic polymer that is commercially available with a wide range of high impact polystyrene (HIPS) applications in various fields because of its mechanical properties. So natural fiber reinforced thermoplastic polystyrene composites can be used as natural wood substitutes in housing, and constructions purposes. Herein we used sugarcane bagasse as a natural filler for HIPS. Where HIPS was melted and mixing with sugarcane bagasse at different ratios using screw extruder, then the fiber/polymer mixture was molded into standard test samples under pressure and heating. The effect of external effectors such as slight aqueous acidic, alkaline solutions, engine oil treatments or UV exposure on the mechanical properties of the prepared composites was investigated. Additionally, the impact of filler content on polystyrene thermal stability and morphological characteristics, water absorption, and swelling capacity were studied.