BPM system is essential for the operation of SASE FEL facilities. Among various types of BPMs
such as the button-type BPM or the stripline-type BPM
only the cavity BPM has the potential to achieve a higher beam position resolution. The amplitude of the TM110-mode excited by an off-center beam in the BPM cavity yields a signal proportional to the beam displacement and to the bunch charge; and the signal phase gives the direction of the displacement. This paper discusses the fundamental characteristics of cavity BPM
and reports our MAFIA simulations and RF measurement of an BPM cavity.