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Diagnostic knowledge derived from dissolved gas analysis (DGA) is complex, imprecise and may lack information about the actual condition of power transformer. To deal with this uncertainty, Dempster-Shafer (DS) Theory is often utilized for the fusion of evidence obtained from various DGA diagnosis. In the Dempster-Shafer theoretic approach, several rules for combining the evidence were proposed, amongst them is unified combination rule by Inagaki. Unified combination rule unifies various types of combination rule from Yager's combination rule to extrapolation combination rule through the introduction of functions. The combination of belief structures derived from power transformer's DGA evidence is implemented through Inagaki's unified combination rule. Expected monetary value was utilized for determining the most optimum combination rule of the unified combination rules. Furthermore, we proposed maximax and maximin criterion for selecting the best decision policy for the mapping of belief structures from transformer's condition spaces to system operator's decision making spaces.