Parallel Kinematic Machine (PKM) is a new type of machine tool, which has the potential to fill in the gap between traditional CNC machines and industrial robots, due to its flexibility and superior motion dynamics. Stiffness is an essential property of a machine tool, as it will affect the machining capability. Although much research has been conducted on stiffness modelling and analysis, most of them employ simplified models and gravity effects have not been well considered or characterized for PKMs. To fill in the knowledge gap, this paper introduces a new experimental stiffness measurement method considering the effect of gravity on the machine tool. An experimental procedure was developed in order to separate the gravity effects on stiffness from the machine structure.