The unconfined compressive strength is one of the influencing parameters that are used for determining the in situ strength of soft, fine-grained soil deposits. Since many previous research works have highlighted the influence of pore fluid type, electrolyte concentration, pH and valence of the pore fluid on unconfined compressive shear strength. The present study has inferred the effect of pore size distribution (PSD) on unconfined compressive shear strength (UCS) of bentonite and kaolinite minerals. It is observed that the pore size distribution of bentonite is a bimodal distribution representing both interpore and intrapore, whereas the kaolinite mineral exhibits a unimodal distribution representing only the interpore. The interpore represents the water molecules bounded between soil aggregates, whereas the intrapore represents the water molecules bounded within the soil aggregated and on the clay surface. From the obtained UCS value of bentonite and kaolinite minerals, it can be inferred that the UCS strength variation of bentonite mineral is strongly influenced by the water molecules bounded on the clay surface or diffused double layer water, whereas the UCS strength of kaolinite minerals is controlled by the net attractive force between the clay particles. The present study demonstrated that the pore size distribution is also one of the parameters that strongly influences the unconfined compressive strength of the soil. © Springer Nature Singapore Pte Ltd 2019.