An experimental investigation was conducted to observe nucleate boiling modes and their characteristics during pool boiling of subcooled water on very small wires. Different nucleate boiling modes, or large bubble and explosive small bubble boiling, were clearly and repeatly observed using a high-speed photography technology, and their coexistence was described and discussed for nucleate boiling, Dissolved air was found to play an important role in large bubble boiling mode and coexisting nucleate boiling. The heat generation characteristics also showed a great difference for different boiling modes. From the observation of the transition process from the large bubble boiling to explosive small bubble boiling, the equilibrium current and associated phenomena were experimentally investigated, and they all had very close relationship with the different boiling modes. Heat transfer model were also proposed to explain the phenomena and measured results, and the heat flux of the coexistence boiling had linear relation with the temperature.