This paper describes the 30 year experience of the Faculty of Agriculture of Chiang Mai university in the development of a participatory extension approach to promote sustainable development of agriculture in Northern Thailand and the extension of the approach to neighboring countries in the Mainland Southeast Asia. It begins with the historical development of system approach to lowland agriculture in Chiang Mai valley when the Faculty had put its emphasis on agricultural intensification in a government irrigation project begun in early 1970s. Conventional extension method was adopted to introduce the improved rice-based, multiple cropping systems for the local people. Many Aimproved@ technologies had been packaged to replace traditional technologies which were assumed to be inferior in agronomic as well as economic criteria. Later, these assumptions were found to be inadequate to explain problems and constrains to technological adoption and diffusion. Furthermore, farmers= perception, knowledge and local capability have helped them to intensify and diversify traditional rice-based multiple cropping systems in the Chiang Mai valley on a large scale without any impact of Agreen revolution@ technological pack. A system approach was introduced in early 1980s to review the overall research and extension activities. A set of criteria was used to assess the performance of alternative (improved) cropping technology including productivity, stability, sustainability and equitability. These criteria >helped research and extension workers 10 modify the extension approach toward farmer-focussed, participatory approaches. With experiences in rural extension and highland development in Northern Thailand, the Faculty of Agriculture has developed programs for the region, especially the mountain areas of the mainland Southeast Asia. This program is expected to introduce a participatory approach for the development of sustainable land use and community forest management as the key to rehabilitating degraded hill areas.