Osteoarthritis (OA) is a common joint disorder and estimated to cause symptoms in 20-40% of the elderly population. 532 nm laser is much absorbed in developed vascular network in the spongy bone which is one of the main characteristics of OA. In this study, a photoacoustic (PA) imaging system with 532 nm laser and 50 MHz US (ultrasound) transducer was developed. Normal and OA knee joints were observed by US and PA imaging. PA signal from the spongy bone was strong where US reflection was very weak. PA signal from the spongy bone was significantly strong in OA compared with that in normal knee while US showed similar low intensity echo in normal and OA. Detailed structure and information on vascular density of spongy bone in rat knee joint were successfully obtained with US / PA combined imaging. US / PA imaging should be useful for early diagnosis of OA.