The electron spin resonance (ESR) spectra of mixed LB films of a conjugated dimer-type Cu-porphyrin and cadmium arachidate show a drastic change by adding a small amount of n-hexatriacontane as trigger molecules that cause a distinct change of orientation of porphyrins. Similar phenomenon has been observed in monomer-type Cu-porphyrin LB films. The marked difference between these cases is that the dimer films show a septet Cu hyperfine structure instead of a quartet of the monomer films with the total spectral span unchanged. This results from the Heisenberg spin exchange interaction between two Cu spins in a dimer, providing direct evidence for the existence of the dimer in LB films. The line-shape analysis of the ESR spectra observed at the X and K bands yields the detailed molecular orientation. The dimer plane is slightly more inclined toward the film plane compared with monomers in the presence of n-hexatriacontane.