In order to elucidate the structural change of Li x CoO2 with temperature (T), powder X-ray diffraction measurements have been carried out using a synchrotron radiation source in the T range between 300 and 90K for the samples with x = 1.02 , 0.60, 0.56, and 0.53. The samples with x < 1.02 were prepared by an electrochemical reaction in a non-aqueous lithium cell. The x = 1.02 and 0.60 samples are in a rhombohedral phase ( R 3 ¯ m ) in the whole T range measured. On the other hand, the x = 0.56 and 0.53 samples exhibit a structural transition around 140K, although the both samples are in a monoclinic phase ( C 2 / m ) down to 90K. That is, the angle between a M - and c M -axis ( β M ) increases monotonically down to 150K, then increases more rapidly with further lowering T. The values of Δ β and a M / b M , which are parameters to characterize a monoclinic distortion from the hexagonal symmetry, are Δ β > + 0.6 ° and a M / b M < 1.732 above 140K, while Δ β < + 0.6 ° and a M / b M ≈ 1.732 below 140K. This suggests that the monoclinic distortion below 140K is mainly caused by a gliding along the basal plane.