Abstract: The Yandonggou eolian loess profile, with the thickness of 120 m located on the north bank of Huanghe river in Lanzhou,deposited on a typical piedmont surface(i. e. the Gansu period piedment surface). Geomagnetic study on this profile shows that the B/M boundary is placed at the depth of 30 m descendingly; the post-Jaramillo and Jaramillo events are situated at the depth interval of 34.5-35.5 m and 41.5-53.5 m respectively, and the Cobb. Mnt event which was discovered in succession recently lies at the depth of 72 m in this profile. The stable positive polarity features of the lowest three samples reflect that they have stepped into the olduvai event period. Estimated combining with deposition-rate, this loess profile deposition began at about 1.7 Ma B. P. This is the oldest eolian loess profile obtained upto now in Longxi basin.