The FWD is widely used to evaluate elastic moduli of pavement layers by means of the back-analysis whilst it is still difficult to analyse deflection basins on unbound layers: the response of unbound materials departs father from the basic assumptions of the elastic theory; basin deflections measured on unbound layers away from the center of the plate decrease faster than Boussinesq theory would predict indicating a lower shear transfer capability of the material. In order to evaluate the reliability of FWD tests on granular base and subbase courses, FWD measurements have been performed in various test conditions; moreover the influence on the results of both the test conditions and the dynamic behaviour of soil have been analysed. For this purpose, static plate load tests have been performed on the same point of the FWD measurements and attempts have been made in order to find relationships between moduli calculated by the two procedures. The found relationships allow to draw useful considerations.