Pi(Spleen, 脾) corresponds to central earth and is called as the basis of acquired essence as it has the function of transforting and transforming the nutrients, keeping the blood circulating, and nourishing the muscle and limbs. Recently many studies to research the nature and deficiency of Pi are actively carried out. Especially functional deficiency of Pi(脾虛證) which occupies 60 to 70% or 88% in miscell aneous disease is divided into deficiency of Qi(脾氣虛), Yang(脾陽許), and Yin(脾陰虛), and tought to be positive when three or more such symptoms as anorexia, abdominal distension, loose bowels, pale face and weakness are present. Investigating the resent studies on the nature of the deficiency of Pi shows that Pi has the function of digestive system and also should be considered as functional unit of mult system related immune, metabolic, hematic, muscular, rnddocrine and nervus system. Various experiments as ptyaline activity test and xylose absorption test are used as an indication to deficiency of Pi and would be helpful to understand its nature. As deficiency of Pi appears in many disease and has various manifestations, further studies to diagnose the symptom of Pi using various experiments and oriental medical diagnostic method should be ensued.