This study describes the selective synthesis of C-36-dimer fatty acids, by reacting unsaturated fatty acid at 533 K for 6 h in the presence of montmorillonite and lithium chloride as the catalyst and cocatalyst, respectively. The catalytic performance of different montmorillonite and organic modified montmorillonite catalysts was investigated in the dimerization of technical grade unsaturated fatty acid, and the influence of the catalyst structure and composition on the catalytic performance was analyzed. All the samples were characterized by X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetry (TG), and dispersion experiments. The results show that the layer spacing and dispersion property of the montmorillonite crystal have important effects on its catalytic performance and the yield of the product. In the dimerization experiments, the maximum yield of the dimer acid is found to be 70.15%, while the lowest yield is 32.37%. It is thought that the larger interlayer spacing in the montmorillonite provides more reaction space for the unsaturated fatty acid, allows the generation of a large volume of dimeric molecules, and helps the product molecules to diffuse out from the layered structure.