The agent-based simulation is a powerful tool to study complex systems. It allows to take into account different levels of granularity, as well as the heterogeneity of the entities composing the system. One of the main issues raised by these simulations concerns the design of the agent behavior. Indeed, when the agent behavior is led by many conflicting needs, desires and necessities, its definition can be very complex. In order to address this issue, we propose a new formalism to define the agent decision making process. This formalism is based on the belief theory, which is a formal theory about reasoning that allows to manage information incompleteness, uncertainty and imprecision. An application of the approach is proposed in the context of a model dedicated to cropping plan decision-making.