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The effects of grain size on the moulding properties of River Niger sand (Onitsha deposit) bonded with Ukpor clay has been investigated in this research. The samples were prepared by washing and sorting to remove impurities and other foreign objects. Weighed samples were sieved and mixed in a Ridsdale laboratory sand mixer (Serial No: 845). American Foundrymen’s Society (AFS) standard test specimens (50mm diameter by 50mm height) were prepared using Ridsdale laboratory sand rammer and moulding properties such as green compression strength, dry compression strength, green shear strength, dry shear strength, green permeability and mouldability were measured using universal sand strength testing machine (Serial No: M8415), electric permeability meter (Serial No: 872) and mouldability tester. Results obtained showed variations in green and dry strengths (Compression and shear), green permeability and mouldability with particle size of the base sand, clay and moisture contents. Particle sizes 355μm and 500μm produced optimal green and dry compression strengths (33.60 KN/m2 and 322.00 KN/m2). The green Permeability number reduced steadily from 8.00 to 6.80 as the grain sizes decreased from 1000μm to 125μm, while the mouldability increased as the sand grains become finer. The results of moulding sand tests obtained from the produced synthetic moulding sand indicated that the sand could be used in the production of cast iron and non-ferrous alloys castings. The results of chemical analysis of the sand and the clay samples indicated that the sand is of high silica content (89.9%) and the clay is rich in both …