Orientational arrangement of long-chain Fluorescein O322 molecules within the monolayer at the air/water interface induced by the compression process was studied by surface second harmonic generation. Compression does not change the average tilt angle of the chromophore in any significant way, but it induces the alignment of the chromophore along a preferred direction perpendicular to the compression direction, originating the in-plane optical anisotropy. A model has been proposed on the orientational ordering of Fluorescein O322 molecules within the monolayer induced by the compression process.