Theoretical analysis and field monitoring show that lateral vibration has very important effect on casing wear in deep & ultra-deep well drilling.The wear mechanism ofcasing under impact-sliding work conditions has been investigated and many experiments have been completed with a newly developed full-scale casing wear test machine.Test results present that adhesion wear,contact fatigue,and grinding abrasion are the main wear mechanisms under impact-sliding test conditions.The friction coefficient and linear wear rate ofthe casing rise obviously with an increase in impact load.And the larger the impact load,the rougher the worn surface ofthe casing.The linear wear rate decreased slightly but the average friction coefficient increased slightly with an increase in impact frequency under an impact load of2,500 N.Both the linear wear rate ofthe casing and the average friction coefficient increased substantially with an increase in impact frequency under an impact load of4,000 N.Under lower impact load conditions,grinding abrasion and contact fatigue are the main mechanisms ofcasing wear;under higher impact load conditions,adhesion wear and contact fatigue are the main mechanisms ofcasing wear. Theoretical analysis and field monitoring show that lateral vibration has very important effect on casing wear in deep & ultra-deep well drilling. The wear mechanism of casing under impact-sliding work conditions has been investigated and many experiments have been completed with a newly developed full- scale casing wear test machine.Test results present that adhesion wear, contact fatigue, and grinding abrasion are the main wear mechanisms under impact-sliding test conditions. friction coefficient and linear wear rate of the casing rise obviously with an increase in impact load. And the larger the impact load, the rougher the worn wear of of the casing. The linear wear rate of reduced casing the average friction coefficient increased substantially with an increase in impact frequency under an impact load of 4,000 N.Under lower impact load conditions, grinding abrasion and contact fatigue are the main mechanisms ofcasing wear; under higher impact load conditions, adhesion wear and contact fatigue are the main mechanisms ofcasing wear.