This work has the purpose to study the effects of aging in natural water on a composite material of polyester matrix reinforced with sisal fibers. The specimens were produced by manual methods, not requiring high technologic devices or being costly.The polyester matrix used was terephthalic unsaturated polyester resin, whose curing agent was MK (Butanox M-50), in proportion of 0,33% (v/v). The sisal fiber used was obtained at a popular trade center in the city of Belém-PA, the fibers were acquired in natural environmental conditions and without treatments. These materials were used to create specimens in the ASTM D 638M standard, which got under aging process after lining. The water absorption was measured by weighing the specimens during the aging.The samples were submitted for a tensile test after the aging time, evaluating how the process changes the properties of the composite. The results show that the increasing water absorption by the composite during the aging process decreases the tensile resistance and the elasticity of the specimens.