The hygroscopic properties of inorganic salt particles, including (NH4)2SO4, NaCl, Na2SO4 and NaNO3, are investigated using a self-assembled hygroscopic tandem differential mobility analyzer (H-TDMA) system. The iso-GF (growth factor) curves are derived to illustrate the effects of the initial particle size (D0) and relative humidity (RH) on the GFs. For those salt particles of 100 nm, the GFs measured agreed well with their theoretical Köhler curves. In the size range of 20-200 nm, the GFs of (NH4)2SO4, NaCl and Na2SO4 particles all continuously decrease with D0 increasing below the deliquescence RH (DRH). However, when RH is higher than the DRH, the GFs of those salts aerosols increase with D0 throughout the investigated size range. Similar increase trend of GFs with D0 is also observed for NaNO3 aerosols though they do not exhibit the abrupt deliquescence behavior. From iso-GF curves, it can be clearly observed that the GFs of (NH4)2SO4, NaCl and Na2SO4 particles all increase with the RH while the values decrease with D0 below DRH. And above DRH, the GFs are more sensitive to D0 for particles smaller than 60 nm, while the GFs are more sensitive to RH for particles larger than 80 nm. For NaNO3 aerosols, the iso-GF curves indicate the size-effect becomes more prominent on their hygroscopicity as the RH increases. The iso-GF curves provide a lucid and explicit insight into the hygroscopic growth of salts particles. Through iso-GF curves, we can clearly elucidate the major factor that affects the ultimate particle diameter at ambient atmosphere.