A theory which can easily be used to discuss the competition between ferromagnetic order and the Kondo effect is presented in this paper. The magnetic order parameters of conduction and localized electrons as well as the hybridization term describing the Kondo effect are introduced simultaneously in a Kondo lattice model using the functional integral method. At zero temperature, quite a complete phase diagram of ferromagnetic and paramagnetic (Kondo) states is obtained in the static saddle-point approximation. At finite temperatures, the dependences of the Curie temperature and the total magnetization on the exchange-coupling strength |J| for various concentrations of conduction electrons, are also given by the theory. The main results are in qualitative agreement with experimental observations in heavy-fermion ferromagnets.