Let p be an odd prime and r a positive integer. Let GR(p 3 , r) be the Galois ring of characteristic p 3 and cardinality p 3r . In this paper, we investigate the self-dual codes over GR(p 3 , r) and give a method to construct self-dual codes over this ring. We establish a mass formula for self-dual codes over GR(p 3 , r) and classify self-dual codes over GR(p 3 , 2) of length 4 for p = 3, 5.2010 Mathematics Subject Classifications: 94B05