Abstract Coulomb blockade and negative magnetoresistance of ultrasmall ferromagnetic tunnel junctions are studied based on the Feynman path integral approach. It is shown that the change in magnetoresistance is considerably enhanced in the Coulomb blockade regime. This is due to the nonlinear current–voltage characteristics caused by the higher-order tunneling processes which set in since negative magnetoresistance tends to make Coulomb blockade unstable. Results obtained are qualitatively in good agreement with recent experimental findings. Coulomb blockade and the magnetoresistance in ultrasmall ferromagnetic double junction are also discussed briefly.