Abstract: A union joint for jointing pipes comprises a union head having a male screw thread formed on the outer peripheral surface of an axial inner end portion thereof and provided at its end with a seat member made of a non-ferrous material, a union tail piece having a collar on the outer peripheral surface of the inner axial end thereof and provided at its end with a sealing surface constituted by a part of a sphere having an arcuate axial cross-section, and a union nut provided at its front inner portion with a female screw thread and at its rear portion with an inwardly projecting jaw. An annular groove is formed in the axially inner end portion of the union head adjacent to the inner periphery of the union head, the annular groove having a substantial axial depth and receiving a substantially ring-shaped seat member, the inner wall of the union head defining the annular groove being knurled and flared at at least the end portion thereof over the entire circumference thereof and caulked onto the seat member.