Density assessment of Sawah Tambang Formation, Ombilin Basin has been done using three-dimensional compute tomography (3D CT) density log approach. Laboratory testing of reservoir core, as well as their evaluations in Central West Sumatra is expected to be better support of core rock analysis process so that it can assist in increasing national oil production. The aim of this research is to determine the direct relationship between the CT number and the grain density of the core. Research method performed measurement of 3D CT scan in the whole core to visualize the 3D core image. The 3D visualize core used to determine the proper location of side wall plug, so the damage cause by the core plugs minimalized. The core side wall plug CT number has been measured and then the grain density calculated. The calculation of grain density derived from permeability, which is measured from the gas permeameter. The result of this two-process cross-plotted and generating the empirical model of the relationship between the CT number and the grain density by using the linear regression. The result of this research showing the empirical model of the sandstone and shale separated in the cross plot between the grain density and the CT number. Linear equation of this relationship divided into two equation which is y = 1835.6x - 3089.7 for the shale zone and y = 1819.9x - 3195.9 for sandstone zone. This linear regression formula used to estimate the grain density value based on the CT number.