Hydraulic transport studies with coal and copper ore have been carried out in a 0.0254 m and a 0.019 m diameter pipeline to observe the effect of solids concentration, surface characteristics and particle size distribution on pressure drop. The coarse coal showed a lower pressure drop than the fines at any velocity of suspension, whereas copper ore, exhibited the opposite trend. It has been observed, that the pressure drop for hydraulic transportation of bi-dispersed particles is a minimum at certain weight proportions of coarse to fines, as compared to that for the transportation of either of the single size. Based on the experimental results, an empirical expression was developed for the prediction of pressure gradient.