This paper presents the effect of moisture on fatigue properties of untreated and NaOH-clay-treated sisal fiber-reinforced epoxy and polypropylene (PP) composites. Sisal fibers were reinforced in epoxy and PP composites at the loading level of 0 to 50 wt.% and the result showed that the water mass uptake continuously increased as the fiber content continuously increased in composites. The treated sisal fiber composites show reduced water mass uptake than that of untreated sisal fiber composites. The fatigue life cycle was affected due to water content in both treated and untreated composites; however, amount of reduction of fatigue life cycle was lower in treated sisal fiber composites than that in untreated sisal fiber composites. An improvement in the order of about 10(2) fatigue life cycles was observed in treated sisal fiber composites over untreated sisal fiber composites (in water medium). This mechanism of reduction in fatigue life cycle due to water content is examined by scanning electron microscopy and dynamic mechanical analysis.