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We have performed new wide-field photometry of the young open cluster NGC6231 to study the shape of the initial mass function (IMF) and masssegregation. We also investigated the reddening law toward NGC 6231 fromoptical to mid-infrared color excess ratios, and found that thetotal-to-selective extinction ratio is Rv = 3.2, which is very close to thenormal value. But many early-type stars in the cluster center show large colorexcess ratios. We derived the surface density profiles of four member groups,and found that they reach the surface density of field stars at about 10',regardless of stellar mass.  The IMF of NGC 6231 is derived for the mass range 0.8 -- 45 Msun. The slopeof the IMF of NGC 6231 (Gamma = -1.1 +/- 0.1) is slightly shallower than thecanonical value, but the difference is marginal. In addition, the mass functionvaries systematically, and is a strong function of radius - it is is veryshallow at the center, and very steep at the outer ring suggesting the clusteris mass segregated. We confirm the mass segregation for the massive stars (m >~8 Msun) by a minimum spanning tree analysis. Using a Monte Carlo method, weestimate the total mass of NGC 6231 to be about 2.6 (+/- 0.6) x 10^3 Msun. Weconstrain the age of NGC 6231 by comparison with evolutionary isochrones. Theage of the low-mass stars ranges from 1 to 7 Myr with a slight peak at 3 Myr.However the age of the high mass stars depends on the adopted models and is 3.5+/- 0.5 Myr from the non- or moderately-rotating models of Brott et al. as wellas the non-rotating models of Ekstr\"om et al. But the age is 4.0 -- 7.0 Myr ifthe rotating models of Ekstr\"om et al. are adopted. This latter age is inexcellent agreement with the time scale of ejection of the high mass runawaystar HD 153919 from NGC 6231, albeit the younger age cannot be entirelyexcluded.

The Initial Mass Function and the Surface Density Profile of NGC 6231