Poly(sodium glutamate) (DP = 160) was modified by sheep brain L-alpha-phosphatidylethanolamine with about 65% substitution in side chains via the active ester method. The structure of the modified polypeptide was characterized by its IR and H-1 NMR spectral data and elemental analysis. The pi-A isotherms of the modified polypeptide at different temperatures and pH were measured. Reproducible Y-type deposition, with a deposition ratio of 0.9 Y-type Langmuir-Blodgett films (30 layers) of the modified polypeptide were also prepared, and lamellar structure, together with an average 32 alpha thick monolayer was observed by transmission electron microscopy. The formation of beta-structure but no alpha-helix structure for cast film of the modified polypeptide was observed by circular dichroism spectroscopy. A wide-angle X-ray diffraction pattern investigation for the cast film showed a small peak at 2 theta = 5.7 degrees and a big peak at 2 theta = 20.1 degrees, corresponding to d spacing of 15.5 and 4.4 Angstrom respectively. In addition, investigation of Na+ ion and K+ ion transport behavior for cast film of the modified polypeptide showed that Na+ ion permeating through the cast film was easier than K+ ion, and both Na+ ion and K+ ion permeated through the film easier under acidic conditions (pH 4) than that under neutral conditions (pH 7).