In this research, Fe₃O₄ nanocatalysts were synthesized systematically microwave-assisted. The effectiveness of the synthesized nanocatalysts in reducing viscosity and upgrading heavy oil was evaluated. The nanocatalysts were investigated for their magnetic and electromagnetic properties. The impact of microwave radiation's time and power on the size and purity of nanocatalysts was investigated. The purities in the crystal network of Fe₃O₄ nanocatalysts expanded as a result of reducing microwave radiation time and power due to less heat production. Increased temperature leads to dope NH₄Cl into the Fe₃O₄ nanocatalysts crystal network. At: 1 min and power of 400 watts the most satisfactory results in the size and purity of nanocatalysts. The electromagnetic properties, size, and effectiveness of the synthesized Fe₃O₄ nanocatalysts have been examined to determine the effect of the synthesis method. The performance of Fe₃O₄ nanocatalysts synthesized by co-precipitation and microwave-assisted viscosity reduction and heavy oil upgrading was evaluated and compared. The crystallite size of the Fe₃O₄ nanocatalysts synthesized by microwave-assisted was smaller than that synthesized using co-precipitation. Fe₃O₄ nanocatalysts synthesized by microwave-assisted and the co-precipitation method decreased viscosity by 28% and 23%, respectively. Moreover, Fe₃O₄ nanocatalysts synthesized by microwave-assisted reduced the sulfoxide index and aromatic index considerably more than the co-precipitation synthesized Fe₃O₄ (90% against. 48% and 13% vs. 7%, respectively).