A ball joint uses a single-piece ball seat of hard plastic that has high rigidity, high load durability and elasticity, to prevent premature fracture of the ball seat caused by manufacturing imperfections in the socket of the ball joint. A preload contact surface is resilient supported at the top of the ball seat to enable uniform distribution of preload of the ball to the socket. At assembly of the ball joint, a cylindrical lower portion of the ball seat cylinder is reshaped into an inwardly bowed surface to envelop the lower portion of the ball. A thin annular wall at the periphery of the preload contact surface of the ball seat defines a thin annular space between itself and the upper portion of the ball seat cylinder to provide the preload contact surface with expansion space. Additional elasticity can be built into the ball seat by providing slits at the junction of the preload contact surface periphery and the base of the thin annular wall.