Water is the most common working fluid using as the coolant for most of nuclear power plants due to its low cost, wide availability, and high heat capacity. In the Fukushima Daiichi accident caused by the tsunami following a powerful earthquake on 11th March 2011, the loss of heat sink occurred. Seawater, which had been considered as an alternative coolant before, was eventually injected into the nuclear reactor for more than one week as an emergency measure. The studies about the characteristics of heat transfer with the seawater as the working fluid have received much more attentions after the Fukushima Daiichi accident. The differences of thermophysical properties such as density, dynamic viscosity, and surface tension, between deionized water and seawater play an important role in the heat transfer. In addition, different boiling behaviors exhibit in the boiling process for two types of working fluid. Compared to pure water, the diameter of bubbles in seawater during pool boiling could be smaller than in water. To improve the safety of a nuclear reactor power plant, mechanisms of the heat transfer using seawater as an alternative emergency coolant should be studied thoroughly. In the present research, experiments of pool boiling in an annulus gap with a riser section after the sudden shutdown of the pump are investigated. Heat transfer characteristic of the heated surface using DI water and 3.5 wt% artificial seawater are described. Boiling behaviors at each fluid are compared and studied.