A dynamic time-domain method is developed to backcalculate the layer moduli, damping ratios and thicknesses of asphalt pavements from dynamic FWD test data. The method uses the SAPSI program as a forward routine and a Newton-Raphson method for the backcalculation. The advantage of the time domain approach is that SAPSI can match selected features of the measured time histories directly, and inaccurate measurements at the ends of histories can be ignored. In this work the peak deflections and the time lag between the peak of the load and the peak of the deflections at different sensors are matched. The Newton algorithm used by Harichandran et al. is adopted to perform the backcalculation. The new algorithm is capable of backcalculating the moduli, damping ratios and thicknesses of three-layer flexible pavements reasonably well. Backcalculations based on synthetic time histories generated using SAPSI show excellent stability and accuracy. Backcalculation at a field site in Kansas also yields reasonable results.