The dynamic mechanical properties of treated sisal fiber-reinforced polyester composites fabricated by resin transfer molding (RTM) have been studied with reference to fiber surface modifications, frequency and temperature. The sisal fibers have been subjected to various chemical and physical treatments like mercerization, heating at 100 degrees C, permanganate, benzoylation and vinyl tris(2-ethoxymethoxy) silane to improve the interfacial bonding with isophthalic polyester resin. Results indicated that treatment changed the storage modulus (E'), loss modulus (E '') and damping factor (tan 3) drastically at a wide range of temperature. The E' value increased for every treatment, and is maximum for the composites fabricated by benzoylated-treated fibers. The T-g value obtained from the E '' value showed an increase as compared to untreated fiber-reinforced composites. The alkali-treated fiber-reinforced composites showed lower tan 6 value. Using Arrhenius' equation the activation energy was calculated and found maximum for the composites fabricated by alkali-treated fiber, which shows good fiber/matrix interactions.