We investigated the temperature dependence of the crystalline morphology in linear low density polyethylene by light scattering, small-angle X-ray scattering (SAXS) and oscillating-DSC. Optical anisotropy in the spherulite, defined by model calculation of the Vv scattering pattern, and the order parameter of crystal orientation within spherulite, estimated by sharpness of the Hv scattering profile, increased in the cooling process while they decreased in the heating process. That is, the morphology is thermally reversible. The morphological change with time after the temperature drop or jump was found to be very fast in several seconds. Oscillating-DSC and SAXS results suggest that the disordering in the heating process is caused by melting of thermally unstable thin lamellae existing between the thick lamellae, which are already developed at high crystallization temperature. Thus, the thermal reversibility is ascribed to the thermally unstable thin lamellae; i.e. the thin lamellae are developed fast at wide temperature range in the cooling process and they melt fast in the heating process at the temperature close to the temperature they are developed. Owing to the fast development of the thin lamellae, the crystalline morphology obtained at high temperature cannot be frozen by quenching.