The glacial phase can be formed from supercooled liquid (SCL) in certain systems, which is called liquid-liquid transition (LLT). Revealing the nature of the glacial phase especially in a single-component system is crucial for understanding the LLT process. Here, by using flash differential scanning calorimetry and cold-field transmission electron microscopy, the structure of the d-mannitol glacial phase and the phase transition kinetics between the glacial phase and SCL were studied. We found that the glacial phase is a fluctuating spinodal-like structure. And the glacial phase displays a first-order melting characteristic when it transforms into the SCL. Furthermore, the LLT diagram in the enthalpy-temperature coordinate system is constructed based on the nucleation-growth and spinodal-decomposition transitions. Our findings provide new insight into the LLT process of the glass forming liquids.