Abstract: It is a common way both in China and abroad to determine the collapsibility of loess in the standard oedometer by either single oedometer test or double oedometer test.However, it still remains unsolved whether the first method or the second one should be used and in the soil testing standards compiled by different administrative departments of the Chinese Government different statements can be found. In this paper it is obtained from test results of an artificially prepared structural loess that there exists an unique collapse line which coincides with the compression curve of the same sample but saturated preliminary. Therefore, it has been proved that both single oedometer test and double oedometer test will yield similar result if the tested samples have identical property, but this demand can hardly be satisfied for natural soils because of their heterogeneity. Next, from the analysis of test results of Osaka clay samples, the drawback of the standard oedometer test for natural loess is discussed, and a method of constant strain double oedometer test for determining the collapsibility of structural loess is proposed for practical use.