  We have conducted a search for L subdwarf candidates within the photometric
catalogues of the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey.
Six of our candidates are confirmed as L subdwarfs spectroscopically at optical
and/or near infrared wavelengths. We also present new optical spectra of three
previously known L subdwarfs (WISEA J001450.17-083823.4, 2MASS
J00412179+3547133, ULAS J124425.75+102439.3). We examined the spectral types
and metallicity subclasses classification of known L subdwarfs. We summarised
the spectroscopic properties of L subdwarfs with different spectral types and
subclasses. We classify these new L subdwarfs by comparing their spectra to
known L subdwarfs and L dwarf standards. We estimate temperatures and
metallicities of 22 late type M and L subdwarfs by comparing their spectra to
BT-Settl models. We find that L subdwarfs have temperatures between 1500 K and
2700 K, which are higher than similarly-typed L dwarfs by around 100-400 K
depending on different subclasses and subtypes. We constrained the metallicity
ranges of subclasses of M, L and T subdwarfs. We also discussed the spectral
type and absolute magnitude relationships for L and T subdwarfs.
