The transformation of the molecular arrangement of an arachidic acid (C-20) Langmuir-Blodgett (LB) film was studied with an increase of pyrrole (Py) quantity which we used as guest molecules to the host molecules, C-20. The change of molecular occupation area of a C-20 LB film was estimated by pi-A curves, a quartz crystal microbalance method and friction force microscopy (FFM). It was found that a C-20 LB film can contain Py molecules 1.4 times as large as C-20 molecules. The observation by FFM showed the enlargement of the occupation area of C-20 upon addition of Py, however, the molecules of C-20 do not change the internal structure of the film upon compression. The experimental results show that the lattice constant of a C-20 LB film is easy to control by the quantity of Py molecules contained in the LB film.