Optical nanofiber (ONF) is a special tool for effectively controlling coupling of light and atoms.In this paper,we study the ladder-type electromagnetically induced transparent (EIT) under ultralow power level in a warm cesium vapor by observing the transmission of ONF that couples the 6S → 6P Cs atoms in the presence of a 6P → 8S control beam through the same fiber.The linewidth and transmission of the EIT signal are investigated at different intensities of the control laser.In addition,we theoretically study the nonlinear interaction at the ONF interface using the multi-level density matrix equations,and obtain good agreements between theory and experiments.The results may have great significance for further study of optical nonlinear effect at low power level.