First-principles calculations within the spin-polarized generalized gradient approximation framework are performed on rhombohedral and its Jahn–Teller distorted monoclinic LiMnO2 with different spin configurations. It is found that the Jahn–Teller (JT) effect is driven by high-spin Mn3+ ion and it should disappear in low-spin state. With JT distortion, the initially degenerate eg states is split by a gap of 687meV, which responds for the semiconduction of monoclinic LiMnO2. Based on the analyses of the changes induced by JT distortion in the crystal, electronic structures and chemical bondings, approaches to suppress the JT effect and synthesize rhombohedral LiMnO2 are suggested. At last, the JT effect is decomposed into an electronic and an elastic term.