  Westerlund 1 is the most important starburst cluster in the Galaxy due to its
massive star content. We have performed BVIc and JKs photometry to investigate
the initial mass function (IMF). By comparing the observed color with the
spectral type - intrinsic color relation, we obtain the mean interstellar
reddening of <E(B-V)>=4.19+/-0.23 and <E(J-Ks)>=1.70+/-0.21. Due to the heavy
extinction toward the cluster, the zero-age main sequence fitting method based
on optical photometry proved to be inappropriate for the distance
determination, while the near-infrared photometry gave a reliable distance to
the cluster -- 3.8 kpc from the empirical relation. Using the recent
theoretical stellar evolution models with rotation, the age of the cluster is
estimated to be 5.0+/-1.0 Myr. We derived the IMF in the massive part and
obtained a fairly shallow slope of {\Gamma} = -0.8 +/- 0.1. The integration of
the IMF gave a total mass for the cluster in excess of 5.0 x 10^4 solar mass.
The IMF shows a clear radial variation indicating the presence of mass
segregation. We also discuss the possible star formation history of Westerlund
1 from the presence of red supergiants and relatively low-luminosity yellow
hypergiants.
