  Formation of sub-galactic halos is suppressed in warm dark matter (WDM) model
due to thermal motion of WDM particles. This may provide a natural resolution
to some puzzles in standard cold dark matter (CDM) theory such as the cusped
density profiles of virialized dark halos and the overabundance of low mass
satellites. One of the observational tests of the WDM model is to measure the
gravitationally lensed images of distant quasars below sub-arcsecond scales. In
this Letter, we report a comparison of the lensing probabilities of multiple
images between CDM and WDM models using a singular isothermal sphere model for
the mass density profiles of dark halos and the Press-Schechter mass function
for their distribution and cosmic evolution. It is shown that the differential
probability of multiple images with small angular separations down to 10
milliarcseconds should allow one to set useful constraints on the WDM particle
mass. We discuss briefly the feasibility and uncertainties of this method in
future radio surveys (e.g. VLBI) for gravitational lensing.
