Abstract: The causes of tooth fall-off of insert rock bit were analyzed through experiments. The geometrical condition of insertion of carbide tooth into the cone hole was derived and the methods of defining each physical quantity in the above expression were given. Mechanical criteria such as measures of press fit between carbide insert and cone hole were put forward, by means' of these results the critical condition of the magnitude of interference was determined. This work provides a theoretical and practical method of design for the carbide insert and cone body.