The seismo-ionospheric precursor prior to the Mw7. 9 earthquake near Wenchuan, China, on 12 May 2008 was observed by the FORMOSAT-3/COSMIC satellite constellation. By binning radio occultation observations, the three-dimensional ionospheric structure can be obtained to monitor the ionospheric electron density variation prior to the earthquake. It has been determined that near the epicenter the F2-peak height, hmF2, descends about 25 km and the F2-peak electron density, NmF2, decreases about 2 × 105 el/cm3 around noon within 5 days prior to the earthquake. The integrated electron content decreases more than 2 TECU between 250 and 300 km altitude.