Amongst a number of methods for estimating pore size distribution of rocks, most commonly practiced are mercury intrusion method (MIM) and water vapour adsorption method (WVM). The application of very high pressure required in MIM for studying very small size pores may result in breaking of cementing bonds and crushing of grains in rock specimen. This renders a limitation of MIM for its use to study very fine sized pores in rocks. On the other hand, WVM has shortcomings towards estimating coarse sized pores due to lack of capillary condensation. Thus, any single method in its present form appears to be inadequate to obtain pore size distribution of rock covering full range of pore sizes from very fine to very coarse. The present experimental investigation has been carried out to explore possibility to develop a suitable method or to modify existing method which would be adequate to provide complete pore size distribution of rock for a full range of pore radii. Experiments have been conducted on 10 different rocks (8 sandstones and 2 shales), the rocks selected for this study are such that a full range of pore sizes from a very fine to very coarse is covered. The pore volume and pore surface area distributions for these rocks have been obtained by both MIM as well as WVM. In addition, total pore volume for these rocks has been determined by water saturation method (WSM). The experimental data thus obtained have been analysed and compared to understand the limitations of these methods toward estimating complete pore size distribution of rocks. Two new parameters namely dnon-adsorption factorT and dwater evaporation indexT have been introduced. These parameters which can be determined by simple experiments form the basis of the proposed simple method referred here as modified water vapour adsorption method (MWVM). The method involves combining the relevant data obtained through non-adsorption factor and water evaporation index with that obtained by WVM. The study clearly demonstrates that the proposed MWVM is competent to provide a complete pore (pore volume and pore surface area) distribution of rocks.