A model for the coexistence of p-wave spin-triplet superconductivity (SC) and itinerant ferromagnetism (FM) is presented. The Hamiltonian can be diagonalized by using the so(5) algebraic coherent state. We obtain the coupling equations for the magnetic exchange energy and superconducting gaps through the double-time Green function. It is found that ferromagnetisation gives rise to the phase transitions of p-wave superconducting states or superfluid ^3He.