UBVI and Hα photometry of the young open cluster NGC 2244

New UBVI and Halpha photometry has been performed for the young open cluster NGC 2244. We classified 30 OB stars as being members of the cluster using proper-motion data and spectral types from previous investigators, along with photometric diagrams obtained in this study. We measured Halpha emission strength of the stars by Halpha photometry and set up a selection criterion to select pre-main-sequence (PMS) stars with Halpha emission. Fourteen PMS stars and seven PMS candidates were found using the criterion. In addition, six stars found near the positions of ROSAT HRI X-ray sources were assumed to be PMS stars and the optical counterparts of these X-ray sources. We determined a reddening of [E(B-V)] = 0.47 +/- 0.04 for the cluster and a total-to-selective extinction ratio of R-V = 3.1 +/- 0.2. The distance modulus derived was V-0 - M-V = 11.1. By comparing our photometric results with theoretical evolution models, we derived a main-sequence turnoff age of 1.9 Myr and a PMS age spread of about 6 Myr. The slope of the initial mass function, Gamma, calculated in the mass range 0.5 less than or equal to log m less than or equal to 2.0 could be at (Gamma = -0.7 +/- 0.1).