Layers occur when insoluble liquids meet each other such as water and oil. The monitoring of the interfaces among various liquid layers are of paramount importance for chemistry purifications, liquids storage in reservoirs, oil transportation, and chemical engineering. However, studies for layered liquid detection are limited. Visible examination has been used as a common practice to distinguish liquid layers, which is rough and in most cases hard to operate for chemical processing. In this paper, a long-period fiber grating (LPFG) based optical fiber sensor was investigated to detect the boundaries between layered liquids. The LPFG sensor when placed among the boundary of liquids will respond to the change of the refractive index between various liquid layers. Laboratory experiments showed that the refractive index difference between layers will induce a sudden change of the LPFG’s resonant wavelength if the LPFG sensor is bonded on an object when moving through the layer boundary. The LPFG sensor will have a higher sensitivity to detect the liquid layers when there is a bigger refractive index difference between the layered liquids. With further approval, the LPFG sensors could be potentially use for accurate liquid layer sensing which is highly demanded for chemical processing and liquid storage.