The Falling Weight Deflectometer (FWD) is become a popular device for measuring deflection basins used for backcalculation of layered elastic properties of pavements systems. It was used to study the seasonal variation of asphalt pavements in the severe climatic conditions met in Quebec. FWD tests were carried out on five road sections, located on the north shore of the St-Lawrence River between Quebec and Trois-Riviáeres, and selected on the base of one granular and four cohesive subgrade soils. Each site was generally characterized by four boreholes and visual observations and also equipped with one frostprobe and two piezometers. FWD deflection tests were conducted at regular time intervals over a one year period initiated in November 1993. The backcalculated moduli were determined with the VIEM and VIEMBACK programs developed by the National Research Council of Canada. This paper discussed some of the research findings obtained from three sites. The use of the backcalculation approach for quantifying seasonal variations in subgrade stiffness due to environmental changes is promising. In spite of the fact that the frost depth is more important in the granular subgrade with 2,6 m, the backcalculated subgrade modulus was found constant after the thaw period up to the beginning of the cold time in December. In the other cases with cohesive subgrade and frost depth of 1,6 m, the backcalculated subgrade stiffness have shown a significant reduction upon thawing, followed by a gradual recovery in time. The seasonal effects on long term performance have been evaluated with structural life model simulation. Relative damage factors have been determined in term of fatigue failure and permanent deformation to take into account in the design of flexible pavements. For the covering abstract of this conference see IRRD number 872978.