Numerous theories attempting to explain lubrication of the synovial joint have been advanced over the last century. The synovial joint is widely thought to predominantly involve fluid lubrication. However, explaining the mechanisms of fluid lubrication in the synovial joint is difficult. I think that most important factor is the spinnability of synovial fluid for excellent lubrication of natural syonvial joint. Synovial fluid has been shown to display high spinnability, and such liquids offer excellent lubricative ability. Therefore, I have named this excellent lubrication ability the “Spinnability effect”. As liquids displaying spinnability have been suggested to display a 2-phase network structure, I next examined whether this is the case for synovial fluid. From my observation, synovial fluid does indeed show a 2-phase network structure. My observational technique thus seems useful for the fundamental investigations, diagnosis of rheumatic conditions and development of medicine for orthopedics.