Summary. Photoelectric B and V light curves of close binary system UW Canis Majoris (08.5 If + 0-B) obtained by Doss are analysed. Combining the photoelectric elements and the spectroscopic orbit by Struve et al. absolute dimensions of the system are determined. The mass of the bright primary (08.5 If) component is found to be 19.3M0. and that of the faint secondary to be 23.2M0.. The primary has filled the Roche lobe and it is 1-2 mag overluminous for its mass. The massive secondary component is most likely a main sequence star. Comparison with the theoretical evolutionary models of massive close binary systems undergoing case A of mass exchange indicate that UW CMa is close to the contact stage of evolution.