Abstract: To estimate the attitude of micro air vehicles (MAVs) on utilizing the horizon for steady flying control, horizon is treated as a straight line based on straight line model. According to the linear camera imaging projection model, the relation between the horizon in the world coordinates and image coordinates is set up. After the theoretical derivation, the relation between the line equation parameters and camera attitude angles is obtained in the image coordinates. MAV attitude angles could be calculated with the line equation parameters of horizon. The validity of the method is demonstrated with experiments. Simulation results also show the superiority of calculating the pitch angle using proposed method over that of using sky area ratio method.