The expansion of the industrial needs of isocyanurate type polyisocyanates and the increasing pressure on reducing free toluene diisocyanate (TDI) monomer content necessitate a highly selective catalyst to ensure a lower free monomer content in the product. A catalyst screening was performed in terms of catalytic activity and catalytic selectivity. Of all the common trimerization catalysts, oligomeric Mannich base (OMB), as a special type of Mannich base catalysts, was synthesized and proved to have the most enhanced selectivity and enough activity for the oligomerization of TDI. The influence of temperature and catalyst concentration on its catalytic reaction was investigated by comparing their reaction rate and reaction selectivity. Dodecanol as a compatibility modifying agent was found to be a co-catalyst of OMB, with a synergistic effect on both the reaction rate and reaction selectivity. The oligomerization process was followed and investigated by gel permeation chromatography (GPC) and isocyanate (NCO) back-titration.