Red supergiants (RSGs) are massive Population I stars, the major progenitor of supernovae. With strong stellar wind, RSGs are an important source of interstellar dust, in particularly for the high redshift galaxies in the early universe when low-mass stars have not evolved to the asymptotic giant branch phase. However, the estimation of contribution of RSGs to interstellar dust suffers large uncertainty mainly because there is no complete sample of RSGs in any galaxy. This talk will present our recent work on building a complete sample of RSGs in M31 and M33 using a novel method based on the near-infrared color-color diagram to remove the foreground dwarf stars. Furthermore, the mass loss rate of RSGs and their whole contribution to the interstellar dust in M31 and M33 are calculated with the complete sample. Discussions on the potential exploration of the sample and the relation of mass loss rate with stellar parameters will be presented as well.