Abstract: Effects of relative humidity on surface adhesion force of Si(100) wafer were studied under micro-scale loads using a self-built apparatus.Based on BET adsorption model,a formula was built to calculate adhesion force,which includes the contribution of capillary force of water film and Van der Waals force under different relative humidity.The test results indicated that adhesion force ascended with the relative humidity.Under lower relative humidity(less than 20%),its influence on adhesion force was not significant. When the relative humidity varying from 40% to 80%,adhesion force demonstrated a remarkable increase with the humidity,and reached to a relatively stable value at higher relative humidity of 90%.