L-alpha-Phosphatidylcholine (DOPC, Dioleoyl) monolayers of different initial surface pressures were spread at the sub-phase surface of a home-made Teflon mini-trough. Surface pressure was measured by the Wilhelmy plate method so as to study the spontaneous membrane insertion of non-ionic surfactant Triton X-100. A formula was deduced to approximately give out the final insertion amounts of Triton X-100 under different initial surface pressures. The results showed that the insertion amount decreased as the initial surface pressure increased and the amount included an 'excess' of Triton X-100, which could be used to approximately give out the interaction energy quantity between Lipid and Triton X-100 molecules in the monolayer.