GdBa2Cu3O7-x superconducting thin films, grown on yttrium-stabilized ZrO2 (YSZ) and LaAlO3 (LAO) substrates, were investigated by a new optical interference fringe method. The results show that (i) two sets of interference fringe pattern are observed on the samples grown on LAO substrates when the film thickness is less than 250 nm, one of them coming from the thin film and the other from the substrate; (ii) the fringe patterns vary slightly on different regions of the samples, which means that the perfection of the thin films is non-uniform; and (iii) defects such as grain boundaries and twin lamellae are observed in some samples. The relationship between the fringes and the degree of perfection of the sample is discussed according to the experimental results.