The problem of determining the boundary of convective cores in massive stars (having a mass greater than 10 solar masses) is discussed. A method is developed, which, in the framework of the mixing-length theory, is used to calculate the increase of mass size of convective cores by overshooting, with a reasonable amount of computational effort. Mass loss by stellar wind is taken into account, and evolutionary sequences are computed up to precentral He-ignition stages for original 20, 60, and 100 solar mass stars. Model results show good agreement with observational data for supergiant stars.
