In this work we further proposed that nanobubbles can be regarded as gaseous analogues of microemulsions (or more directly as a special kind of microemulsions), through replacing oil in microemulsions with gas. Thus, the whole nanobubblecontaining solution is in a two-phase coexistence state: the gas-in-water nanobubble phase coexisting with the external gas phase. The stability mechanism for bulk nanobubble can be summarized as follows: The shrinking of large bubbles and the adsorption of amphiphilic molecules lead to the formation of a compressed amphiphilic monolayer at the interface of bulk nanobubbles. The compressed monolayer is featured with an ultra-low surface tension, and thus it is the curvature free energy, rather than surface tension, that dominates the interface free energy.