The structure transition in cluster-cluster aggregation (CCA) under different external electric fields has been investigated by computer simulations. The aggregates are generated from off-lattice CCA models involving the field-induced dipolar interactions and temperatures by means of a Metropolis algorithm. When the parameter K increases from 0 to ∞, the clusters gradually change from a diffusion-limited CCA to a chainlike pattern, where K stands for the relative strength of the field-induced dipolar interaction with thermal energy. The relation of fractal dimension Df to parameter K can be approximately expressed as Df=DE+(DDLCA-2DE)e-βK with β50.64, where DE and DDLCA stand for the fractal dimensions of the CCA when K-∞ and K-0, respec-tively. This structure transition is a transition between a prototype disorder structure and a relative order one. The transition is attributed to the variation of the dominating interaction of systems from thermal disorder to field-induced dipolar interaction with K rising.