The electron paramagnetic resonance (EPR) spectral data (the g factors and hyperfine structure constants) and d-d transition spectra for the tetragonal Mo5+ centre in [Mo6O19][N(C4H9)4]3 salt are theoretically investigated from the complete diagonlization method (CDM) for a 4d1 ion in tetragonally compressed octahedron.The theoretical results are in good agreement with the experimental data.The dependency of the g factors of the ground state on the R// (Mo=O bond length) has been studied.It is shown that the g factors varied with the R// approximately in a linear way.