The external magnetic field and temperature dependencies of the magnetic structure and antiferromagnetic resonance frequencies in LaMnO3 are studied. A recently developed model which includes orbitally dependent superexchange and single-ion anisotropy interactions is used. The temperature dependence is described in the mean-field approximation in good agreement with experiments. H–T phase diagrams in the main field directions (a, b, c) are drawn. The problem of the origin of weak noncollinearity of Mn3+ spins is solved in our work. Also an optimal method for obtaining an expression of the magnetic free energy is represented.