Langmuir-Blodgett (LB) films were found to detach partially from a substrate passed through the air-water interface. When a piezoelectric quartz plate (quartz-crystal microbalance, QCM) was employed as a substrate, the detached amount could be obtained in nanogram level from the frequency change of the QCM substrate. The removal of LB films mainly occurred when the substrate was lowered slowly into a pure water surface, but not when it was raised. The detachment was largely reduced when the substrate was lowered quickly through a water surface having a low surface tension at low temperature. The detachment was also reduced when well-oriented and defect-free LB films were prepared from long chain amphiphiles or polymerized monolayers.