Moisture sorption isotherms of pure yacon powder (YP) and yacon powder with maltodextrin addition (YP-MD) (total solids ratio of yacon〖DK〗∶maltodextrin=1∶1) were determined at different temperatures (15, 25 and 35℃) by static gravimetric method. The best model for describing yacon adsorption characteristic was determined through seven models fitting analyses. Thermodynamic properties, namely net isosteric heat of sorption, differential entropy, enthalpy-entropy compensation and spreading pressure, were determined from sorption isotherm data of yacon. The effect of maltodextrin (MD) on the sorption characteristics and thermodynamic properties were also discussed. The results show that the sorption behaviour of yacon follows a type Ⅲ isotherm; GAB model is the best fit one for describing yacon adsorption characteristic for both YP and YP-MD. Equilibrium moisture content of YP and YP-MD increases with water activity (aw). Addition of MD decreases the monolayer moisture content (M0) of yacon. When temperature ranges from 15℃ to 35℃, the M0 of YP and YP-MD ranges from 15.04% to 9.04%, 8.80% to 5.91%, respectively. The net isosteric heat of sorption (qst) and differential entropy (Sd) decreases with the increasing of moisture contents. The qst of YP and YP-MD decreases from 67.44kJ/mol to 62.23kJ/mol, and 66.17kJ/mol to 62.12kJ/mol as the moisture content increases from 6.33% to 75.29%. The value of Sd of YP is always higher than that of YP-MD. The spreading pressure of YP and YP-MD increases with increasing aw, and decreases with increasing temperature. The value of spreading pressure of YP is always higher than that of YP-MD at fixed aw and temperature. Through this study, a conclusion can be drawn at last that the driving force of moisture sorption for PY and PY-MD is entropy.