Display omitted ► An air layer is seen on underwater lotus surface without additional pressure. ► The external pressure is a key to sustain “air layer effect” on lotus surface. ► The state of air layer on underwater lotus surface is monitored by contact angles. ► The microstructures on lotus surfaces are important for its air layer underwater. Sustaining an air layer on superhydrophobic surface is known necessary for the applications of the “air layer effect” underwater on basis of previous literatures. However, the air layer becomes unstable when put underwater and even disappears when the hydraulic pressure increases. Thus much attention has been focused on the sustainability of an air layer on a superhydrophobic surface. This article studies the sustainability of the air layer on the surface of a lotus leaf by applying external pressure on the surface, which is a direct method to measure the sustainability of the air layer. It is displayed that the hydraulic pressure is a key factor to sustain an air layer on a superhydrophobic surface. Maintaining a certain amount of hydraulic pressure is necessary to sustain an air layer on a superhydrophobic surface, which may be important for the low drag or friction reduction of a superhydrophobic surface underwater. Therefore, we believe that the micro-designs of superhydrophobic surface which is capable of the “air layer effect” underwater should be connected with hydraulic pressure applied on it.