This paper presents the development of a six-degree-of-freedom (DOF) geometric error measurement (6GEM) system that can be applied to the simultaneous measurement of six geometric error components of the moving axes of a meso-scale machine tool (mMT). The system consists of a laser module constructed by a cube beam splitter and a pigtailed laser diode, three two-dimensional position sensitive detectors (PSDs), and an additional cube beam splitter. The laser module moving with the positioning system of the developed mMT testbed generates two perpendicular laser beams, one of which is further divided into two laser beams at the second cube beam splitter. These three laser beams are detected by the three PSDs, and the full pose of the laser module is then calculated simultaneously by forward and inverse kinematic computations. The calculated full pose of the laser module is translated into six-DOF geometric errors of the mMT testbed. A series of experiments are performed to demonstrate the effectiveness and accuracy of the proposed 6GEM system. The experimental results show that the measurement accuracy of the 6GEM system was better than for translational error components and for angular error components.