Abstract: A method of measuring the critical temperature and pressure of thermally unstable compounds was developed,in which a thick wall borosilicate glass tube was used as sample tube,and a coated quartz glass was Supported by the Tianjin Natural Science Foundation(No.973602911). To whom correspondence should be addressed.used as heater. The advantage of the measuring apparatus was that a small amount of sample was sufficient to determine the critical properties accurately,a very short time was needed to heat the sample,and the fluid of interest could be uniformly heated. Taking n-pentane,n-hexane and 1-heptene as standard reagents,the reliability of the apparatus was checked by measuring the critical temperatures and critical pressures of these three compounds. Critical temperatures and pressures of 4 substances including dipropyl ether,allyl amine,isoamyl nitrite ester,and dimethylsulfide were measured. The low residence time (<20 s) at elevated temperature minimized decomposition and other reactions,making it possible to measure the critical properties of many unstable fluids. Except for dimethylsulfide,all data were determined for the first time.