Pulsars, one of the most accurate radio beacons in the universe, have potential applications in fundamental physics. As the extreme astronomical laboratories, they offer unique opportunities to probe the fundamental physical effects, which are usually not approachable with ground based environments. In this talk, we will focus on pulsar timing and its application. Basic pulsar timing technique will be introduced, and several of related applications, e.g. to detect gravitational wave, to test gravity theories, and to measure cold hadron equation of states, will also be discussed.