Abstract:On 12 May 2008 at 06:28UT a major earthquake of Mw 7.9 struck Wenchuan County(31.0°N,103.4°) in China. Occultation profiles derived from FORMOSAT3/COSMIC between 2007 and 2010, were used to investigate the ionoshperic precursor of the Wenchuan earthquake area and magnetic conjugate area. Relative to the NmF2(F2peak electron density) of 6-17 days(25 April-6 May) prior to earthquake, it has been determined that near the epicenter the NmF2, decreased during 04:00-08:00UT(12:00-16:00LT) within 5 days(7-11 May) prior to the earthquake,and the maximum deviation was about (2-4)×10 5 el/cm 3. Furthermore we choose the NmF2 data derived from 3 years(12:00-14:00LT ,7-11 May,2007-2010, except 2008) as background value. The NmF2 5 days prior to earthquake was lower than background value. The maximum deviation was about (2-4)×10 5 el/cm 3. And there was similar phenomenon detected in magnetic conjugate area.