In this paper we propose a novel face detection method based on the vanishing point of vertical lines in the scene to improve system performance in a common surveillance application. While most existing face datasets and detection techniques are based on the assumption that the camera has a similar height as the target faces, in practical situations the camera may be installed at different heights. Such discrepancy often degrades the detection performance of algorithms based on learning with certain (e.g., frontal) face orientation. In this paper we propose a transformation to rectify face images (video frames) such that it is not necessary to collect training data of different face orientations. Furthermore, with the proposed method there is no need to perform complex camera calibration. The only required information is the vanishing point of vertical lines, which can often be estimated easily. Experiments show prominent improvements in face detection performance can be obtained with the proposed image transformation.