We investigate the structural changes of spinel lithium titanate (LTO; Li4Ti5O12) via the electrochemical Na insertion-extraction cycles, using X-ray diffraction (XRD) and analytical transmission electron microscopy (A-TEM). The XRD reveals that an extra phase with an about 4‑5% larger unit cell volume is generated in pristine LTO during the first cycle. The A-TEM reveals that the extra phase co-exists with Li4Ti5O12 in a single particle and is identified as a Na-substituted LTO phase (Na-LTO; Li4-x Na x Ti5O12, 0 < x < 3). The reversible electrochemical capacity of the electrode (155mAhg−1) is more than 90% of the theoretical one, suggesting that both LTO and Na-LTO are involved in the charge-discharge reactions.