[en] We have studied the Neel temperature of undoped La₂CuO₄ in the orthorhombic state using the quantum Heisenberg model and double-time-temperature spin Green's function theory. In the Tyablikov decoupling approximation, we find that the orthorhombic phase transition is necessary to keep La₂CuO₄ having 3D-AF ordering at finite temperatures. When the interlayer couplings J₁ in the a-b plane and J₂ in the b-c plane are much smaller than the antiferromagnetic coupling J₀ in the a-c layer, the Neel temperature can be expressed in asymptotic form: k_BT_N=2πJ₀/ln[16J₀/(J₁-J₂)]. ((orig.))