PURPOSE: To obtain a dicyclopentadiene resin having a relatively low molecular weight and a relatively narrow molecular weight distribution by heating a reaction material containing a dicyclopentadiene monomer in the presence of a strong acid under specified conditions.

CONSTITUTION: A reaction material containing a (di)cyclopentadiene monomer is heated at a temperature of about 240 to about 320°C for a time sufficient to form a dicyclopentadiene oligomer resin showing a weight-average molecular weight in the presence of a strong acid and in the range of about 260 to about 1,000 and a polydispersity index of about 2.3 or below desirably under the autogenous pressure in a sealed container. The strong acid desirably used is a Bronsted acid selected among hypophosphorous acid, p-toluenesulfonic acid, hydrochloric acid, hydrogen iodide, phosphoric acid, methanesulfonic acid, and a mixture thereof and is present in a concentration of about 0.05 to about 1.0 wt.% based on the total weight of the reaction mixture.