A new scaling theory for fully-depleted double-gate (DG) SOI MOSFET's is established, which gives a guidance for the device design so that maintaining a precise subthreshold factor for given device parameters. By investigating the subthreshold conducting phenomenon of DG MOSFET's, the effective conducting path effect (ECPE) is employed to obtain the natural length lambda(3) to guide the design. With ECPE, the minimum channel potential phi d(eff,min) shows the new scaling factor alpha(3) to model the subthreshold behavior. Compared to conventional scaling rule, our model accounting for ECPE caused by substrate doping density not only provides a unified scaling rule but also offers the basic designing guidance for fully-depleted DG SOI MOSFET's. (C) 2004 Elsevier Ltd. All rights reserved.