Making use of log T e f f − M b o l diagram along with Stohers' stellar evolutionary tracks (1972), we estimated the masses of 107 O stars in HII regions, which were selected from the catalogue compiled by Goy (1973). It is found that our estimated masses of O stars range from 20 M ⊙ t o 120 M ⊙ and about 20% of them falls in the mass range above 60 M ⊙ in agreement with earlier findings of Conti et al. (1975).

Optical thickness of Rosette Nebula for Lyman Continuum is examined with W. Gebel's data. And the effective temperatures of the six exciting 0 stars of the nebula are obtained on the assumption that O stars emit black body radiation. The results are found to be generally in between Spitzers old and revised values.