Abstract: The geysering-induced instability has significant potential impact on nuclear reactor accident transient during natural circulation or gravity-driven reflooding process with low power density and mass flow rate etc. By employing electric heating of 2×2 rod bundles, the geysering flow and boiling characteristics were experimentally studied in a vertical tube with the inner diameter of 32 mm. The instability characteristics of the geysering flow and boiling were investigated and the effects of the heat flux density on the geysering boiling period were discussed. The results show that when the fluid in the channel under certain heat flux density reaches saturated and overheating states, the abrupt eruption of vapor and refilling of subcooled liquid will appear intermittently accompanied with various flow patterns, such as bubbly, slug, churn and annular flows. The eruption period of geysering boiling depends on the delay time of boiling which decreases with the increase of heat flux density. The geysering flow and boiling phenomenon will disappear and change into another type of two-phase flow instability when the heat flux density increases beyond a threshold.