The goal of this paper is to improve the capability of gravity inversion assessment of complex structures using a multiple-source model as indicated by a study of a foothill belt of northwestern Taiwan. In this study, a gravity inversion computer program is applied based on research developed by Tsai, which contains a Marquardt inversion algorithm for mathematical calculations and incorporates constraints on geological parameters. The computed and modified geological parameters are transformed into coordinates. The response of a proper geological model is calculated using the Talwani technique.