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Fluid loss during cementing operation in depleted reservoirs or in deep wells of reservoirs with low breakdown pressure is a major concern for maintaining well integrity. To overcome this problem, the weight of cement slurry is usually reduced. Although various slurry formulations have been proposed during the last decade, the cost and availability of required additives is still an important concern, especially when the oil price is low. The objective of this study is to make lightweight cement slurry with a density of 105 lb/ft 3 , using the combination of natural pozzolan and API class G cement.  In this study, a new formulation based on the optimum amount of a natural pozzolan and other additives is proposed. Based on the results of 24-hour compressive strength test and free water volume, 30% replacement of cement powder with pozzolan was selected as an optimal amount. Addition of more pozzolan into the slurry reduces its pumpability even at room temperature. Bottom-hole condition was simulated by increasing the temperature to 150°F and chemical additives were used to maintain the rheological properties of this slurry. Fluid loss control agent, dispersant and retarder were used at optimum values 0.5, 0.08 and 0.05 (%bwoc), respectively. The compressive strength of the cement rock was monitored at 3, 7 and 30 days, reaching 3528 psi after 30 days. Cited as : Larki, O., Norouzi Apourvari, S., Schaffifie, M., Farazmand, R. A new formulation for lightweight oil well cement slurry using a natural pozzolan. Advances in Geo-Energy Research, 2019, 3(3): 242-249, doi: 10.26804/ager.2019.03.02.