As Web-based online communities are rapidly growing, the agents in social groups need to know their measurable belief of trust for safe and successful interactions. In this paper, we propose a formal model of reputation resulting from available feedbacks in online communities. The notion of trust can be defined as an aggregation of consensus given a set of reputations. The expected trust of an agent further represents the center of gravity of the distribution of its trustworthiness and

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... rthiness. And then, we precisely describe the relationship between reputation, trust, and expected trust through a concrete example of their computations. We apply our trust model to online Internet settings in order to show how the trust is involved in a rational decision-making of the agents.