  Mass loss is an important activity for red supergiants (RSGs) which can
influence their evolution and final fate. Previous estimations of mass loss
rates (MLRs) of RSGs exhibit significant dispersion due to the difference in
method and the incompleteness of sample. With the improved quality and depth of
the surveys including the UKIRT/WFCAM observation in near infrared, LGGS and
PS1 in optical, a rather complete sample of RSGs is identified in M31 and M33
according to their brightness and colors. For about 2000 objects in either
galaxy from this ever largest sample, the MLR is derived by fitting the
observational optical-to-mid infrared spectral energy distribution (SED) with
the DUSTY code of a 1-D dust radiative transfer model. The average MLR of RSGs
is found to be around $2.0\times10^{-5}{\text{M}_\odot}/\text{yr}$ with a
gas-to-dust ratio of 100, which yields a total contribution to the interstellar
dust by RSGs of about $1.1\times10^{-3}{\text{M}_\odot}/\text{yr}$ in M31 and
$6.0 \times10^{-4}{\text{M}_\odot}/\text{yr}$ in M33, a non-negligible source
in comparison with evolved low-mass stars. The MLRs are divided into three
types by the dust properties, i.e. amorphous silicate, amorphous carbon and
optically thin, and the relations of MLR with stellar parameters, infrared flux
and colors are discussed and compared with previous works for the silicate and
carbon dust group respectively.
