The importance and cost of avionics system in the integration of an aircraft is continuously increasing. And we can expect enlarged software portion in the system integration for the more intelligent, reliable, and automated avionics system. Both military and commercial avionics community have moved toward commercial-off-the-shelf(COTS) equipment and open systems architecture not only to increase affordability but also to reduce acquisition cost, shorten development time and risk. The same concept is applied in developing avionics test system used for the avionics system integration test. In this paper, we present important topics in the development of avionics system including real-time operating system, interconnect data bus, software development methodology, software development process, and system integration test.