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Background: This research investigated the effects of various iron fertilizers and the introduction of Piriformospora indica (P. indica) fungus on the biodegradation of diesel oil in soil experiencing water deficiency. Methods: The treatments included applying pure Fe at rates of 0, 60, and 90 kg/ha, derived from Fe sulfate (FeSO4) and Fe slag (containing 58.2 % Fe2O3), in a diesel oilcontaminated soil with concentrations of 0 % and 8 % (W/W). These treatments were implemented in a corn plant cultivation system, where the plants were inoculated with P.indica fungus under conditions of drought stress. After 90 days, the plant and soil Fe concentration was determined by atomic absorption spectroscopy (AAS). Furthermore, the extent of diesel oil bio-degradation in the soil was assessed. Results: Applying 90 kg/ha pure Fe from Fe sulfate and Fe slag significantly enhanced the bio-degradation of diesel oil in the soil by 13.2 and 16.1 %, respectively. However, the efficiency of this process was lower under drought-stress conditions. In addition, Plant inoculation with P.indica significantly enhanced the diesel oil bio-degradation of the soil contaminated with 8 % (W/W) diesel oil under drought stress by 15.7 %. Moreover, the soil microbial activity was significantly increased by 14.1 %. Conclusion: The combined use of P.indica, along with iron slag and iron sulfate, markedly enhances the biodegradation process of diesel oil within the soil. [ABSTRACT FROM AUTHOR]