Abstract: An experimental apparatus was established for determining extremel - low vapor pressure by Knudsen torsion-effusion method. The torsion - effusion constant K of the instrument were experimentally determined by vaporizing n - hexadecane which is chosen as a standard substance. To check the reliability of the K values, vapor pressures of 1 - octadecene were measured and compared with the values selected. Vapor pressures of 5 esters including dimethyl adipate, diethyl adipate, dibutyl maleate, diethyl sebacate, dipentyl phthalate were determined at temperatures from 293 K to 373 K and correlated by the Antoine equation.