Abstract: Based on the theory that borehole breakouts are caused by the shear failure of the borehole wall in the direction of minimum horizontal principal stress, this paper discusses the problem of determining the direction of the horizontal principal stress from borehole elongation.The measuring method of four-arm diameter log, data processing and the principle of identification of breakout are discussed, and then two different kinds of examples are given.Finally, the records obtained from four-arm diameter log of 29 oil-well in the Pu Yang area, Henan province are analysed. It turns out that the mean orientation of breakouts in this area is N 15E and the orientation of maximum horizontal principal stress determined is N 105E.