In oil and gas well cementing, a robust cement sheath is required toensure long-term integrity of the wells. Successful completion of cementing jobhas become more complex, as drilling is being carried out in highly deviated andhigh pressure-high temperature (HPHT) wells. To gain desired compressive strength and rheological properties has become hard and complex task in HPHT wells. The use of nanomaterials inenhanced oil recovery, drilling fluid, oil well cementing and other applications isbeing investigated. Being smaller in size and having a high surface area, nanomaterials can provide solutions to challenges faced in HPHT cementing. This paper presents the results of an experimental studyconducted to investigate the effect of nanoclay as an additive, on themechanical and rheological properties of Type-G cement slurry under HPHTconditions. Nanoclay based cement mixes were prepared by replacing cementwith 1%, 2% and 3% BWOC nanoclay, and admixed with silica flour, and otherchemical admixtures. The cement slurries were prepared and cured at 290°F temperature and 3000 psi pressure condition as per API 10-B. Compressive strength and rheological properties were measured after conditioning the slurry at 194°F temperature and atmospheric pressure conditions as per API standard. It is observed that addition of nanoclay at1% BWOC in cement slurry, enhances the compressive strength as compared to base mix without nanoclay and other percentages of nanoclay (2% & 3%) BWOC. Further, it is investigated that nanoclay addition enhances the reological properties of cement slurry that could help in mud displacement depending on requirement.