An ultrasonic guided wave-based cure monitoring technique is developed to estimate cure-dependent anisotropic viscoelastic properties of carbon fiber reinforced plastics (CFRPs). The guided wave propagating in the transverse direction of a CFRP was measured during a cure process and the energy velocity and attenuation were obtained by signal processing. A micromechanics model and transfer matrix method were used to express the energy velocity and attenuation as functions of only the complex Young's modulus of the resin. The complex Young's modulus of the resin was estimated from the measurement results using response surfaces of the relationship between the energy velocity, attenuation and the resin complex modulus. It was verified that the estimated storage modulus and loss modulus showed reasonable values and behavior. The development of the anisotropic viscoelastic property of CFRPs can be fully determined in real time using this technique.