Effect of solute adsorption on vapor pressure osmometry measurment was studied by four low molecular weight organic compounds and five narrow MWD oliomeric polystyrene samples with known average molecular weight in Knauer VPO apparatus. Correction factor for solute adsorption was proposed

which leads to that the instrumental constant K does not depend upon the molecular weight of the solute. The accuracy of measuring number average molecular weight of low molecular weight polymer was improved.