Abstract: Drilling jar is a essential stuck freeing device. Fracture may be happens to drilling jar’s joint because of bad working conditions and complex load. Study on the fracture of drilling jar’s joint and corresponding improvement measures carried out on joint can lift joint quality and prolong it’s service life to reduce fracture cases. Fracture failure research was carried through tests and theory based on a fracture accident of a drilling jar’s top joint. Macroscopic analysis, dimension measurement and nondestructive testing were done for failure drilling jar’s top joint. The material’s chemical and physical properties (including chemical composition, mechanical properties, metallography) were checked. The drilling strings assembly was analyzed specially and the bending strength ratio between drilling jar’s top joint and adapter joint was calculated detailedly. The results show that the drilling jar’s top joint is broken by virtue of fatigue induced by irrational drilling strings assembly and improper matching between drilling jar’s top joint and adapter joint. The joint’s fatigue life was reduced further owing to material’s impact toughness less than standard value. The improvements were made at the end of this passage and good results had been obtained.