Lamellar-to-isotropic phase transition is observed in the system of TX-100/n-C8H17OH/H2O induced by neutral water-soluble polymer poly(ethylene glycol) (PEG) with molecular weight ranging from 400 to 20 000. The location of PEG in the lamellar liquid crystal and the microstructure change of the lamellar phase during phase transition are investigated by means of H-2 NMR, small-angle X-ray diffraction (SXRD), rheology, polarized optical microscopy (POM), and freeze-fracture transmission electron microscopy (FF-TEM;). Calculations based on the "swelling model" show that 0.92-2.58 wt % PEG2000 penetrates into the amphiphile layer and the rest resolve in the water layer. Both of these two kinds of locations induce the lamellar-to isotropic phase transition. The longer the chain length of PEG, the higher the efficiency is. In addition, a critical molecular weight of PEG is observed before phase transition occurs, with which the disturbance of PEG on the microstructure of lamellar liquid crystal is most prominent. The critical molecular weight of PEG is independent of the thickness of water layer. The value is 2000 for the system of TX-100/n-C8H17OH/H2O.