A simple and practical approach to calibrate extrinsic parameters of camera mounted in intelligent vehicle is presented. Because mapping of three parallel lines in the world into image plane have same vanishing point and different slope, according to perspective projection principle, an inherent relationship exists among camera extrinsic parameters and vanishing point and slope. Such the analytic expression of camera extrinsic parameters can be built by mathematical derivation, without iterative and optimizing calculation. After positioning intersection points and slopes of imaging lines manually or automatically, extrinsic parameters can be calculated directly. The experimental results show that the method can be applied in various circumstances.