Abstract: Based on the analysis of the composition and the characteristics of a 3-D separated sheet, it is found that the 3-D separation phenomenon can be described in terms of either Maskell's mode or the Lighthiil's mode. However.it must be pointed out that the Ma-skell's mode can only be used for describing an open separation, whereas the Lighthill's mode for a closed separation, ft is also shown in the paper that the separated line for the open separation starts at a normal point, where the streamline is tangential to and leaves from the surface of a body.

Key words: three-dimensional separation, mode, open separation, separation