Introduction: Upekkha is mainly understood as a neutral attitude of the mind which means an inner harmony or balancing quality up to the present. However, when the Pali nikaya is reviewed in detail, there exist more varied and flexible meanings of upekkha, for example inner calmness, pure consciousness, ignorance, satisfaction, detachment, neutral feeling, observation without subjective conception, wisdom free of discrimination and so on. Body: Especially, the Upekkha in the stage of 4th rupaloka-jhana, which belongs to the group of jhana-Upekkha, plays a role of impartial insight and inner calmness. In the stage of 4th rupaloka-jhana, the Upekkha is most purified and reinforced together with sati. When the sati penetrate the phenomenon of body and mind arising, staying and disappearing course in the rupaloka-jhana, this most purified and reinforced Upekkha is used for the impartial insight supporting sati. Like this, upekkha and sati [sampajannana] have a closing relationship, supporting each other in the jhana. For the study, I first examine the use of Upekkha as found in Pali-nikaya, and then classified them according to their function and characteristic. Second, I investigate the dynamic relationship between Upekkha and jhananga. Finally, I examine the relationship between Upekkha and sati in the 4th rupaloka-jhana. Conclusion: Throughout the study, we can learn about that owing to the most purified and reinforced Upekkha in the stage of 4th rupaloka-jhana, the stage of 4th rupaloka-jhana is best condition for the insight among the rupaloka-jhana, and then Upekkha is as important quality as sati in Buddhist practice.