Heat source is one of the four important parameters in geothermal systems of reservoir layers, caprock layers, and fractures. Its existence will cause a very significant difference between the aquifer layer and the geothermal reservoir layer. Unfortunately, the most geothermal issue is focused on fractures and clay cap layers since the MT (magnetotelluric) method is sensitive to response to the conductive clay cap. The MT and gravity method are two powerful geophysical methods to implement in geothermal exploration. The gravity data mostly used to prompt either structure or reservoir layer. Therefore, the gravity data here will be used to map the geothermal heat source in the Mt. Lawu geothermal system. The tilt angle or tilt derivative method is applied to delineate the area and determine the depth of the heat source. The vertical derivative value of gravity obtained from upward continuation with h = 200, 400, and 600 m above sea level. The result shows that the depth of heat source in the Lawu geothermal system is estimated to be at a depth of 900 - 2896 m below sea level in the southern part of the study area. These hot rocks degrade from the South toward the Northeast of the suspected caldera zone. There is also an indication of a fault that allegedly connects the reservoir layer and the fumarole manifestation zone in the Northeast study area. Structures in this area are looking likely dominated by a normal fault and among them form a graben structure.