Abstract: An ultrasonic technique was employed to in situ monitor the cure behavior of glass and carbon fiber reinforced vinyl-ester (VE) resins. The longitudinal sound velocities and at tenuations in the pure VE system, the glass fiber/VE composite system and the carbon fiber/VE system were investigated. The ultrasonic measurements show that the changes in the gel time, bulk storage modulus and activation energy of VE resins by the presence of the fiber fabrics can be observed. By comparison with the pure VE resins, the activation energy and cure reaction rate of the glass fiber/VE system become less (14 % cut down) and fast, respectively. However, the activation energy and cure reaction rate for the carbon fiber/VE system become large and slow. In sum, the ultrasonic technique as a non-invasive , in-situ and real time measurement technique can be used to study the curing behavior and processes of fiber reinforced composites.