The image of lint containing foreign fiber features that the background (cotton fiber) is homogeneous and has a normal gray-level distribution; the object (foreign fiber) is smaller, darker than the background but its gray-level distributes is a wide range. In this paper, a Background Estimation Thresholding(BET) method is presented to segment the objects from such kind of images. The segmented objects will be used to determine the existence and measure the quantity of foreign fiber. The experimental results show that the BET is effective and fast, and can be used in the online foreign fiber inspection in volumes of lints.