  We compiled a list of about 250 SX Phoenicis stars known in Galactic globular
clusters in order to study period-luminosity relation for this type of
variable. The absolute magnitudes of these stars are derived using
metallicity-luminosity calibration for RR Lyrae stars. The mixture of different
radial and non-radial modes present in SX Phoenicis stars and the lack of
unique method of mode identification cause the difficulties in defining strict
period-luminosity relation. As a solution we propose to use confirmed
double-mode radial pulsators.
