Natural fibers have being used on manufacturing of composite materials for car components. Between natural fibers, sisal and piassava deserve special attention. These components can be exposed to many environmental conditions, for example humid and saline environments, present in littoral regions of coastal countries.This study aims to evaluate the effects of salt water on the properties of a polyester matrix composite material reinforced with piassava natural fibers; and for this, using cheap and easily accessible materials and equipment.The polymeric matrix used was terephthalic unsaturated polyester resin, the curing agent was MK (Butanox M-50), in proportion of 0.33% (v/v). The fiber used was piassava, obtained at the 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 638 M standard, which got under aging process in salt water.The water absorption was measured by weighing the specimens during the aging in determined times. After their respective exposure time the samples passed through a tensile test to evaluate their properties and compare the effects of different exposure times.The results show an effective decrease in properties of elasticity and tensile resistance by the composite, directly proportionally to the aging time. It's possible to conclude that the salt water absorption degrades both fiber and resin.