Abstract: Coseismic ionospheric disturbances(CID)obviously exist when large earthquake occurs by analysis of Mw7.9 Wenchuan Earthquake and Mw9.0 Japan Earthquake.The duration of CID is short,usually starts 30~40 min before earthquake and reaches the extreme value about 10 min after earthquake.Then it disappears.The duration and amplitude of CID depends on seismic magnitude,specifically,magnitude greater and the duration and amplitude greater.In addition to clear precursory positive anomaly of ionospheric TEC around the earthquake epicenter,it’s mentionable that weak negative anomaly of ionospheric TEC exist far away from the earthquake epicenter.