PURPOSE: To prepare the compound in a high yield by adding a specific mono- alcohol to the ojganic diisocyanate prior to or during the trimerization reaction to obtain polyisocyanate having isocyanurate and allophanate groups.

CONSTITUTION: The polyisocyanate containing mono-isocyanurate and mono- allophanate groups in a molar ratio of the former to the latter of 10:1-1:5 and having low viscosity and excellent compatibility with polar solvents and reactive isocyanates is obtained by catalytically trimerizing a portion of the isocyanate groups of an organic diisocyanate having (cyclo)aliphtically bonded isocyanate group in the presence of a catalyst [preferably, especially N,N,N-trimethyl-N(2- hydroxyethyl)-ammonium hydroxide, etc.], adding 0.01-0.5 moles, per mole of the organic diisocyanate, of a mono-alcohol containing 6-9 carbon atoms(e.g. 2-octanol) to the organic diisocyanate prior to or during the trimerization reaction and finally terminating trimerization reaction at the desired degree of trimerization by adding a catalyst poison and/or thermally deactivating the catalyst.