The purpose of this paper is to provide a unified framework to investigate the meanings of decision principles. A decision principle is a guiding rule to define a preference order over the set of alternatives for a given decision problem. Although many decision principles have been proposed as rational ones, it is uniformly agreed that no decision principle is uniformly best and it is an important subject for decision theory to explore the meaning of a decision principle to specify when it is reasonable. This problem has been considered in various ways. This paper, first, characterizes & rational decision principle as a rule which satisfies the two conditions, Pareto consistency and the similarity condition. Pareto consistency has been accepted as an essential condition for a rational decision principle. The similarity condition, which is an original idea of this paper, requires that if two decision problems are structurally similar, then their preference orders induced by a rational decision principle should be similar. Decision principles are, then, represented in the most natural way as functors from the category of decision problems into the category of ordered sets. We demonstrate that almost all the decision principles proposed as rational satisfy this formulation and that furthermore, the max-min principle is completely characterized by these two conditions. This final result shows that the similarity condition is an essential property of rational principles and the present category-theoretic formulation can be a unified framework for the study of decision principles.