Abstract: The non-invasive longitudinal ultrasonic transmission technique was employed to in-situ monitor the curing behavior of the vinyl-ester resin in the molds with different thicknesses. The gel time，storage modulus and the curing reaction degree of resin were measured by the longitudinal ultrasonic wave technique. The results show that the ultrasonic transmission signals can reliably reflect the curing reactions of vinyl-ester resin. The gel time，changes of storage modulus and the reaction degree with the curing time can be in-situ achieved from the variations of velocities and at tenuation of longitudinal ultrasonic waves. The ultrasonic technique is a useful tool for non-invasive，in-situ and real time investigation of curing reaction processes of resin-based composites.