The improvement of critical current densities of superconducting wires is essential for commercial applications. Many approaches have been demonstrated to enhance flux pinning of YBa2Cu3O7-delta (YBCO) second-generation (2G) superconducting wires and therefore to increase critical current densities of the materials. In this work, we study the microstructures which lead to enhanced critical current density through flux pinning by nanoparticles in metal-organic deposited (MOD) YBCO films with Dy or Zr additions. Transmission electron microscopy (TEM) observation revealed that large densities of nanoparticles can be formed in YBCO films through altering the precursor stoichiometry and cation additions. Most of these nanoparticles are in the range of 10-50 nm, and are well dispersed, making them ideal for flux pinning in YBCO 2G wires. The effects on critical current of these nanoparticles acting as flux pinning centers are investigated. (c) 2007 Elsevier B.V. All rights reserved.