Liquid has been widely believed to have a random homogeneous structure, but recent studies have shown that liquid can have local structural order. This local structural ordering allows even a single-component substance to have more than two liquid states. Liquid-liquid phase transition (LLT) between different liquid states is of fundamental importance to understand the physical nature of liquid, and thus has attracted much attention. A molecular liquid, triphenyl phosphite (TPP), has been known to have two distinct amorphous states. However, the nature of the newly found apparently amorphous state called “glacial phase” has been a matter of debate for a long time, primarily because it usually contains nanocrystals. Thus, some researchers thought that the glacial phase is just an exotic solid state formed by nanocrystals. To solve this controversy, we applied a fast differential scanning calorimetry (DSC), whose scan rate is faster by 10⁴ times than conventional DSC, and succeeded to suppress nanocrystal formation and observe the transformation process between two pure liquid states including its reversible process. Our study unambiguously shows the first-order nature of the transition and provides firm evidence that the transition is LLT.