Natural fiber/thermoplastic composites were made using aspen fiber and polypropylene. Several different levels of aspen fiber (30 to 60% by weight) were used with and without the addition of a compatibilizer (maleic anhydride grafted polypropylene, MAPP). These composites were tested for strength properties, exposed to boiling and cyclic liquid water and oven drying tests, and evaluated in an accelerated weathering test for 2000 hours. As the level of fiber increases without MAPP, flexural strength, flexural modulus, tensile strength and tensile modulus increases but notched and unnotched impact strength decrease as compared to pure polypropylene. The presence of MAPP has a very large positive effect on flexural strength, flexural modulus, tensile strength and tensile modulus as compared to composites made without MAPP. As the level of fiber increased in the composites, the rate and extent of thickness swelling and moisture sorption increased as compared to pure polypropylene. The presence of MAPP decreased the extent of swelling and moisture pickup. Weathering of composites under accelerated weathering conditions (water sprayed for 18 minutes followed by 102 minutes without water spray of UV radiation produced by a 6500-watt xenon are light source) was restricted to the outer 0.5 mm of the surface. Weathering gave rise to the development of a white chalky surface layer which started at about 150-200 hours of weathering. After 2000 hours of accelerated weathering, weight loss increased with increasing fiber content and there was more weight loss in specimens containing MAPP as compared to specimens without the addition of MAPP. After scraping off the chalky white layer after 2000 hours of weathering, weight loss was greatest in specimens containing no fiber.