In order to check the effect of structure of VPO on the instrument calibration factor two kinds of VPO

one with stand-up type thermistor and the other with hang-down type

have been calibrated with a number of identical standard samples. Including nine organic compounds

with molecular weight ranging from 178 to 1089 and six polystyrene samples from 2.82×103 to 4.42×104. The results show that the calibration factor of the instrument with hang-down thermistors is dependent on the molecular weight of solute while that of the instrument of stand-up thermistors is inde-pendent on the molecular weight of solute. Some factors including the concentration and viscosity of the solution the quantity and shape of the liquid on the thermistors have been discussed.