Geological structure is one of the important factors that should be recognized before drilling. Several geophysical methods have been applied to identify the structure. For gravity data analysis, there are First Horizontal Derivative (FHD), Second Vertival Derivative (SVD) and Euler Deconvolution (ED), that frequently applied in many geothermal fields. Accordingly, this study try to apply the FHD method to locate contacts of density contrast from gravity data that usually associate with the present of the structures. However, before applying the method directly to real data, 3-D forward modeling of gravity data is built to learn the ability in detecting and characterizing the structure. Starting from the simplest model through the complex model, the gravity responses from CBA result and also FHD then are analyzed. At the end, a complex model based on the real case in Blawan-Ijen geothermal area is also reconstructed. From the real data, CBA anomaly shows high gravity anomaly which might be associated with the existence of several volcanoes in this area. Moreover, the maximum gradient anomaly from FHD result coincided with the presence of the structures obtained from RS analysis and geological study.