Poly(3-dodecylthiophene) monolayer was self-assembled on the gold surface by immersion of gold substrate into the solution of polythiophene. The peak of the photoluminescence (PL) spectra of the monolayer sample was redshifted compared to those of a bulk polythiophene. By exciting the sample with linearly-polarized light incident obliquely to the sample, strong anisotropy in the fluorescence intensity was observed as the polarization direction of the pump beam was rotated. From this result, the tilt angle of the thiophene ring with respect to the surface normal of the gold substrate was measured.