Irradiation experiments were carried out by the Comitato Nazionale Energia Nucleare to evaluate the mechanical properties of the fuel pin for fast reactors. The fuel pins were placed in-pile in an irradiation device specially equipped for the continuous measurement of the fuel dimensional variations by means of a resonant cavity system; total power and cladding temperatures were also measured. The data consist mainly of the measurement of the axial gap variations versus linear heat rating and burnup for the three types of fuel under study: pellets, cored pellets, and vibro-compacted sol-gel. The important movements in the fuel column and the different behavior of the three types of fuel tested were observed.