The phase transitions of Langmuir films of the liquid crystals (LCs) 4’-n-pentyl-4-cyano-biphenyl (5CB), 4’-n-heptyl-4-cyano-biphenyl (7CB), 4’-n-octyl-4-cyano-biphenyl (sCB) and 4’-n-decyl-4-cyano-biphenyl (10CB), have been investigated by a Maxwell displacement current (MDC) measuring technique. A phase transition of LC molecules from the planar surface alignment phase to the polar one was clearly detected in the range of low surface pressures during monolayer compression. A phase transition between a stable monolayer phase and a stable three-layer phase of the liquid crystal film was detected during further monolayer compression.