Unmanned aerial vehicles (UAVs) are widely used for commercial and military purposes. UAVs are mostly dependent on an Inertial Measurement Unit(IMU) for navigation. We have presented an alternate approach to estimate the states of an UAV using only onboard camera which can be either used alone or assimilated with the IMU output to enable reliable, accurate and robust navigation. As UAVs are generally fitted with cameras, they present information rich sensor alternative for obtaining useful information from the craft, with the added benefits of being light weight, small and no extra payload. Assuming a pinhole camera model and perspective projection, the first approach is based on optical flow and horizon detection. In the second method, the craft state is estimated using differential and discrete epipolar constraint with planar constraint assuming the scene is slowly moving. Algorithms results are compared with real time data collected from the IMU.