The changes of Li+/vacancy arrangement in Li2+x Ti3O7 with a ramsdellite-type structure upon topo-electrochemical Li+ insertion were investigated by the entropy measurement of reaction combined with the Monte Carlo simulation. The experimental entropy measurement was conducted by potentiometric and calorimetrical methods. The obtained experimental data were in good accordance with simulated results. The results indicated that the ordered Li+/vacancy arrangement appeared at the compositions of x ∼0.45 and ∼1.20, where the observed entropy of reaction humped. The ordering of Li/vacancy were also indicated at the composition x ∼0.24 and 1.16 in Li2+x Ti3O7 by the Monte Carlo simulation which considers the most stable Li/vacancy arrangement in terms of Coulombic interaction. This substantial agreement between electrochemical behaviors and computational results confirmed that the formation of superstructure arising from Li/vacancy arrangement during the electrochemical reaction deeply related to the atomic level Coulombic interactions.