Antiplasticization, defined as an increase in glassy modulus of polymers upon dilution with low molecular weight compounds, was studied for highly crosslinked epoxy resins. A series of oligomers with average molecular weight 500–4000 was incorporated with epoxy resins for elucidating the effect of their molecular weight on antiplasticization. The antiplasticization was found to be dependent on the molecular weight of the oligomers used. It was most evident when the lowest molecular weight oligomer was incorporated, and seems to be suppressed by increasing molecular weight of the oligomers. These results were discussed in correlation with the behavior of dynamic mechanical properties and the changes in specific volume as a function of the epoxy resin and diluents.