In situ observation of crystal growth process of YBCO superconductive oxide under an external electric field has been carried out. Nucleation and growth processes of Y123 (YBa2Cu3O7-x) crystals from liquid + Y211 (Y2BaCuO5) on MgO (100) substrate through a peritectic reaction were observed. A DTA system with in situ observation and external electric field imposition mechanism was specially designed for measuring the peritectic temperature. It was found that the equilibrium peritectic temperature was 10 12 +/- 0.5 degrees C, which was almost the same as reported in literature, and an external electric field did not affect the peritectic temperature. However, an external electric field retarded nucleation process of Y123 although it did not affect the growth kinetics. The retardation resulted from an increase of critical Gibbs energy for nucleation due to an external field. (c) 2006 Elsevier B.V. All rights reserved.