The pendant drop technique has been used to compress and expand the monolayer film,both the surface density of surfactant molecules and surfactant orientation on the surface of a pendant drop can be determined with polarized fluorescent image for the surfactant molecules which has a chromophore-The experiment on octadecyl Rhodamine B(ODRB) molecules shows that:(1) The surface density of ODRB follows the formula σ/σ0=(D0/D)2 in the compressing process-(2) The ODRB molecules are absorbed on the surface of pendant water drop in the way of lying flat-(3) The pendant drop method is a reliable and convenient technique for compressing the surface density of surfactant molecules-