In getting an overall viewpoint of critical heatflux (CHF) boiling mechanisms, first of all, we should have the awareness that the CHF triggering mechanism associates with the flow pattern development very much. For the subcooled flow boiling, due to the existence of two kinds of flow pattern, two kinds of CHF mechanism are considered existing. A criterion for determining the flow pattern development is introduced and popular hypothetical CHF mechanisms are overviewed for the two kinds of flow pattern respectively.