In this paper gasification performance of Mazut of Iran's refineries based on Gibbs free energy minimization is studied. A parametric study was performed in order to investigate the effects of different agents (oxygen, air, and steam), equivalence ratio and gasifier pressure on syngas composition, gasification temperature, high heating value, cold gas efficiency, and exergy efficiency. The results of oxygen-gasification show that Mazut can be employed in the integrated gasification power generation systems and provides constant and appropriate exergy efficiency contents. Additionally, mazut gasification with air-steam or oxygen-steam is more efficient in order to produce syngas for chemical applications. The present study assesses syngas compositions and gasification performance characteristics for different operating conditions.