Hygroscopic behaviors of some inorganic salts with various sizes were measured at different relative humidity（RH) by a self-made Hygroscopic Tandem Differential Mobility Analyzer（H-TDMA) system at 25℃. For（NH4)2SO4，NaNO3，NaCl and Na2SO4 particles with a diameter of 100 nm，a good agreement were observed between the measured hygroscopic growth factors and the theoretical Kohler curves. At a RH below deliquescence point，the growth factor decreases linearly with increasing of the particle size，the smallest particle appearing a highest growth factor.