  We have been monitoring M31 with HRC and ACIS onboard \chandra regularly in
the past two years. By combining eight \chandra ACIS-I observations taken
between 1999 to 2001, we have identified 204 X-ray sources within the central
$\sim 17\arcmin\times17\arcmin$ region of M31, with a detection limit of $\sim
1.6\times10^{35}$\lum. Of these 204 sources, 21 are identified with globular
clusters, 2 with supernova remnants (one of them is spatially resolved with
\chandra), and 8 with planetary nebula. By comparing individual images, about
50% of the sources are variable in time scales of months. We also found 14
transients. Combining all the available transients found in literatures, there
are 25 transients in M31 and M32 detected by \chandra and \xmm; we present some
of the long-term lightcurves by using the HRC, ACIS and \xmm data. The spectral
shape of 12 sources is shown to be variable, suggesting that they went through
state changes. The luminosity function of all the point sources is consistent
with previous observations (a broken power-law with a luminosity break at
$1.7\times10^{37}$\lum). However, when the X-ray sources in different regions
are considered separately, different luminosity functions are obtained. This
indicates that the star formation history might be different in different
regions.
