A new hybrid measurement technique was developed and employed to study line-averaged three-phase holdup distribution in a Slurry Bubble Column Reactor (SBCR) using gamma-ray densitometry (GRD) and point optical fiber probe (OFP) techniques. The OFP technique was used to measure the local gas holdup at 45 nodes in the cross-section area of the SBCR at three different axial levels. Thus, the GRD technique measured the spectrum of the SBCR for two different orientations simultaneously with the OFP technique at the same operating conditions to determine the solid holdup distribution radially and axially. The line-averaged solid holdup for 25% solids loading on the higher measurement level at the column's center decreased compared to the lower level by 45.7%, 43.6%, and 41.7% using superficial gas velocities 0.2, 0.3, and 0.4 m/s, respectively. These results are highly remarkable as it often assumed that the solids exhibit a uniform distribution inside the slurry phase.