How to implement software Agent is a key problem to develop Agent-oriented programming languages and development tools. Aiming to solve this problem based on the mainstream OO (object oriented) technology, this paper first discusses the differences between Agent and object, and then presents an implemental architecture and behaviors decision algorithm of software Agent. The architecture and related algorithm are based on OO technology and the simplified improvement of BDI (belief desire intention) Agent model. An OO-based design framework of software Agent is also proposed by using the POAD (pattern-oriented analysis and design) method. The approach is helpful to clarify how to extend the OO method suitably for developing Agent-oriented programming languages and development tools.