Ultraviolet-visible (UV-vis) absorption spectroscopy has been employed to investigate surface pressure and number of monolayer dependences of molecular aggregation and orientation in Langmuir-Blodgett (LB) films of the title compound (abbreviated as octadecyl-TCNQ). It has been found that the LB films consist mainly of species with stacked structure of TCNQ chromophore and that the percentage of the species changes little with the surface pressure and number of monolayers. Tilt angles of the TCNQ chromophore in the LB films have been calculated from the measurements of their polarized W-vis spectra. The results show that the orientation of the TCNQ chromophore depends on the number of monolayers and surface pressure.