The present study reports the effect of additive on pressure drop and rheological characteristics of fly ash slurry at high concentration (above Cw60% by weight). Pressure drop has been measured using a pilot plant test loop for fly ash slurry at various concentrations with and without additive. Mixture of sodium carbonate and Henko detergent has been used as a additive. Rheological studies are also carried out for fly ash slurry at the same concentrations. By using the rheological data, pressure drop has been predicted in a straight pipeline of 40mm diameter at higher concentrations. Experimental results obtained from a pilot plant test loop were compared to the analytical results based on the Darby and Melson (1981) algorithm. The comparison showed very good agreement between these sets of data, and it is found that there is reduction in pressure drop when additive like soap solution is added to the fly ash slurry at higher concentrations.