The study of speech acts in multi-agent system is very interesting and important. Discussed in this paper are speech acts of pragmatics from the perspective of establishing practical reasoning agents. Now the research on speech acts in MAS (multi-agents-system) focuses on three aspects: 1) the ontology of speech acts; 2) the mechanism for an agent to deduce reasonable speech acts; and 3) the mechanism for an agent to correctly deal with speech acts performed by other agents. The three aspects are different but closely interconnected. This paper focuses on the aspect 2) and aspect 3), and formalizes the model with LOBA (logic of believable agents). Considering the model of agents' cognitive processes, a series of cognitive elements are discussed, which include perception, belief, emotion, desire, goal, intention, and commitment, and the dynamic relations among these elements are analyzed by introducing corresponding cognitive actions. On the basis of the above work, this paper describes how agents generate and deal with speech acts. As for LOBA, it expands the work of KARO and LORA logic, treats agents' emotions and cognitive actions as modal operators, and interprets cognitive actions which only occur in agents' brains by a three-layer model. By virtue of these measures, LOBA logic can describe the practical reasoning processes of agents with stronger expressivity than ever.