In this study, the relationships among plasticity ratio, clay fraction and liquidity index of clay-sand mixtures were investigated. Two types of clays, namely, kaolin (K) and calcium-montmorillonite (CM) were used to prepare clay-sand mixtures of different grain size distributions. Clays were mixed with clean sands of two different grain size distributions (well graded sand-SW and poorly graded sand-SP) at clay contents ranging between 0% to 100% by increments of 10%, in terms of dry weight. Fall cone, Casagrande, plastic limit and laboratory vane shear tests were performed on various clay- sand mixtures with different clay contents. Experimental results revealed that, above 100% water content, undrained shear strength values of CMSW and CMSP mixtures were determined to be around 50 kPa. On the other hand, undrained shear strengths of KSW and KSP mixtures are around 30 kPa, for a water content of 30%. Average undrained shear strengths of CMS and KS mixtures at their plastic limits are obtained as 210.68 kPa and 119.62 kPa, respectively. Average undrained shear strengths of these mixtures at their liquid limits are determined to be 2.40 kPa and 1.88 kPa, respectively. It is concluded that various mechanisms control liquid and plastic limits of clay-sand mixtures which are influenced by gradation of sand, clay content as well as origin of clay. As a result, several relationships among parameters including water content, liquidity index, undrained shear strength, cone penetration depth, fall cone liquid limit and Casagrande liquid limit were presented.