In this work, the structural and magnetic properties of LaMn_1-xCo_xO₃ (x = 0.00, 0.25, 0.50, 0.75, 1.00) are investigated. The structural characterization of compounds by X-ray powder diffraction is evidence for a rhombohedral structure (R -3c space group). Much less increase of the unit cell volume suggests that Co enters in lattice as Co⁺² for x ≤ 0.5 while much more decrease of the unit cell volume suggests that Co enters in lattice as Co⁺³ for  x < 0.5. The results of magnetic measurements show strong ferromagnetic interactions Mn⁺⁴ - Co⁺² in the range (0.0 < x ≤0.5) while antiferromagnetic interactions Mn⁺³ - Co⁺³ and Co⁺³ - Co⁺² is progressively settled at higher cobalt concentrations.