A model is suggested to treat nematic block copolymers with mesogenic groups in the main chains. It is based on a mean-field approach taking into account attractive interactions between chain segments, and allows the description of chains with varying flexibility (with somewhat flexible mesogens). The free energy is calculated; it yields the behaviour of the temperature-dependent nematic-isotropic phase transition in the system for various sets of parameters (concentration of the two components and constants of anisotropic interaction).