The dynamic elasticity of linoleic acid monolayer on a subphase of10~(-4)mol/L TbCl_3 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 theresulting polymerized monolayer is even smaller than that of its corresponding monomermonolayer. The elasticity of the polymerized linoleic acid monolayer decreases with in-creasing polymerization time. The explanation based on entropy has been presented.