An on-line monitoring system was proposed to monitor lubrication oil real-time condition. Oil is the machine equipment blood, and its condition has very serious influence on machine running. Firstly, this paper expiated the research significance of on-line oil monitoring. Then the key problem of on-line oil monitoring was discussed and its goal was analyzed. Furthermore, basic research content and research thread were put forward. Based on the modern photoelectric detection, microscopic imaging and computer image processing technology, an on-line acquisition system of lubricating oil particle image was proposed. This work provides a new research idea for on-line monitoring system which is important to develop the technology of oil condition monitoring.