The free electron laser (FEL) is a fourth-generation light source based on the interaction between electromagnetic fields and ultra-relativistic electron bunches which travel along the axis of a vacuum beampipe. The Cavity Beam Position Monitor (Cavity BPM) is a new kind of diagnostic tool for measuring the beam displacement from the beampipe axis. At the “Sincrotrone Trieste S.C.p.A.”, Italy, a seeded FEL, presently under development for the FERMI@Elettra project [1], will provide an ultra bright coherent X-ray light which will be employed for the study of chemical reactions and dynamic molecular processes. This paper presents the Cavity BPM and a new measurement system, with the first resolution assessment.