The dynamic elasticity of linoleic acid monolayer on a subphase of10-4mol/L TbCl3 at various surface pressure has been measured by means of dynamic oscil-lation method in measuring the change of surface pressure caused by periodic compression-expansion cycles of the barrier. The elasticity of monolayer increases with increasing ofsurface pressure linearly. The linoleic acid polymer monolayer has been obtained underUV-irradiation in situ when keeping a constant surface pressure. But the elasticity of the resulting polymerized monolayer is even smaller than that of its corresponding monomer monolayer.The elasticity of the polymerized linoleic acid monolayer decreases with increasing polymerization time.The explanation based on entropy has been presented.