This article reports the design, development, and evaluation of an online software application for assessing students' understanding of curricular content based on concept maps. This computer-based assessment program, called the Assessment Agent System, was designed by following an agent-oriented software design method. The Assessment Agent System is composed of five types of software agents: instructor agent, student agent, management agent, assessment agent, and reporting agent. Software agents in the system, through communication and cooperation, collectively provide the functionalities of user-system interaction, user management, task authoring and management, assessment delivery, task presentation, response collection, automatic assessment, and reporting. One-to-one evaluations and group evaluations were conducted to reveal students' perceptions of the Assessment Agent System. Measures of visual clarity, system functionality, consistency, as well as error prevention and correction indicate that the Assessment Agent System is a useful tool for large-scale assessment based on concept maps. Through the process of design, development, and evaluation of the Assessment Agent System, this study demonstrates the agent-oriented approach for producing educational software applications. Furthermore, this research explored the concept map assessment method for the Assessment Agent System. When node terms and linking phrases are provided, the assessment of student concept maps can be conducted automatically by comparing student concept maps with the criterion concept map, proposition by proposition. However, the validity of the proposition-comparing method depends on the accuracy and thoroughness of the criterion propositions. Therefore, assessment criteria need to be continually refined and improved through the examination of student-created propositions.