The purpose of this study were to evaluate and compare the balance ability at different conditions in normal 20 to 29 years old on unstable platform, KAT 2000(Breg, Inc., Vista, CA. 1994). Static and dynamic BI(balance index) were measured 3psi and 5psi surface conditions. Static tests were done on right and left leg separately, then both legs together with the feet apart 20cm with the eyes opened and closed. Dynamic tests were done on both legs together with apart 20cm with the eyes opened. A dynamic test was performed in which the subject moved platform in a circular manner to chase a moving object on a computer screen. Seventy healthy students(average 21.6 years, male, female) were tested. In this study applied the paired t-test and correlation to determine the statistical significance of result. The results were as follow: 1) The mean static balance index of the Rt leg was $119.9{pm}75.72$ on 3psi surface condition with the eyes opened, and that of the Lt leg was $224.3{pm}121.16$. 2) The mean static balance index of the Rt leg was $93.1{pm}24.16$ on 5psi surface condition with the eyes opened, and that of the Lt leg was $180.5{pm}61.76$. 3) The mean static and dynamic balance index of both legs were $76.4{pm}31.86$, $2187.6{pm}696.99$ on 3psi with the eyes opened, and $68.3{pm}14.82$, $1938.7{pm}525.41$ on 5psi respectively. 4) The mean static balance index of the Rt leg was $517.8{pm}220.87$ on 3psi surface condition with the eyes closed, and that of the Lt leg was $588.6{pm}204.81$. 5) The mean static balance index of the Rt leg was $271.9{pm}192.151$ on 5psi surface condition with the eyes closed, and that of the Lt leg was $363.4{pm}98.97$. 6) The mean static balance index of both legs was $332.6{pm}137.31$ on 3psi surface condition with the eyes closed, and that of the 5psi was $288.5{pm}133.07$. 7) The balance index on 3psi surface condition was significantly higher than that of 5psi (p<0.05, p<0.01). 8) The balance index with the eyes closed was significantly higher than that of the eyes opened (p<0.05). 9) The balance index on the left leg was significantly higher than that of the right leg (p<0.05, p<0.01). 10) There was no correlation between static balance index and dynamic balance index. 11) Therewas no correlation between weight or height and balance index.