The power transformer is one of the most expensive pieces of equipment in a high-voltage AC power system. An adequate monitoring system for power transformers can help in reducing the failure rate and thereby enhance system reliability and economic efficiency. Long-term and accelerated aging is not possible in real transformers, so prorated models of real transformers are preferred. This paper presents a study aimed at the aging of different cellulose papers and pressboards impregnated with transformer oil subjected to accelerated thermal aging at an oil temperature of 120 $^{\circ}$C and winding temperature of 150 $^{\circ}$C for time intervals extending from 0 to 500 h. The study manifests a close relationship between the degree of polymerization and the tensile strength of paper and pressboard that could be used in predicting the condition of solid insulation at various elevated temperatures. The graphical representation of various solid insulation properties and their correlation analysis has also been carried out.