Magnetization measurements of the YBa 2 Cu 3−x Sn x O 7 superconductor were performed at 4.2 K in magnetic fields up to 29 T. The critical current densities are deduced from irreversible magnetization and the pinning forces are evaluated. X-ray diffraction and EPMA analysis indicated that the sintered YBa 2 Cu 3−x Sn x O 7 (0 ⩽ x ⩽ 0.2) consists of superconducting YBa 2 (Cu,Sn) x O 7 (123) phase and a small amount of Y 2 BaCuO ? (211) and CuO phases. Tin dissolves largely in the 123 phase. no new phase was formed. Sn-doped YBa 2 Cu 5 O 7 appears to have a higher critical current density than the undoped compound at 4.2 K. This is considered to be due to the substitution of Sn for Cu in the 123 lattice, and the finely dispersed 211 particles in the 123 phase result in an increase of the pinning force.