A grain size control technology will be described. YBa 2Cu 3O 7− δ thin films are grown by a pulsed laser deposition method in 400 mTorr oxygen atmosphere. The films are grown by a self-template method with various self-template growth temperatures. However the temperature of the main YBa 2Cu 3O 7− δ layer was fixed. Grain size of YBa 2Cu 3O 7− δ thin films on MgO substrates are observed by atomic force microscope. Mean diameter of grains are calculated by the number of grains in scanned area of 1×1 μm 2 . The mean diameters are 2.49×10 2, 2.89×10 2, 4.047×10 2 and 4.192×10 2 nm for films with an initial layer growth temperature of 670, 690, 714 and 730 °C, respectively. Grain size is found to depend on the initial growth stage of the YBa 2Cu 3O 7− δ thin film growth. These grain sizes have a strong relationship with the inverse of substrate temperature indicating thermally excited migration of YBa 2Cu 3O 7− δ nuclei. It is also clarified that migration length of YBa 2Cu 3O 7− δ nucleus is longer on an YBa 2Cu 3O 7− δ films than on MgO substrates.