The liquid–vapor two-phase region has attracted the attention of various fundamental studies and applications. To explore the liquid–vapor two-phase region, underwater electrical wire explosion (UEWE) is a useful tool. In this work, we observe two different mechanisms of the liquid–vapor transition in UEWE: volume vaporization and spinodal decomposition. The main factors through which the mechanisms occur are the heating power and pressure accumulation in the system during UEWE. From the electrical properties and shadowgraph images, the phase transition process of the exploding wire is analyzed. The effects of possible instabilities during the liquid–vapor phase transition are also discussed. We expect that the thermodynamic properties of the liquid–vapor two-phase region can be explored using the UEWE technique based on the understanding of the phase transition process.