A novel method based on a ball bar to measure the thermal errors of machine tool spindles was proposed to compensate the thermal error of 5-axis Computer Numerical Control(CNC) machine tools. With the help of the movements of the two rotational axes, the volumetric paths comprised of two orthogonal circles or arcs were measured by the ball bar. The least square method was used to process the measurement data to obtain the volumetric positions of the machine tool spindles. Through frequently testing with this method at certain intervals, the thermal errors of machine tool spindles could be calculated, including 1 axial elongation and 2 radial thermal errors. To explain this method, a 5-axis CNC machine tool with a titling rotary table was taken as an example to elaborate the installation, measurement procedure and the identification principle. Finally, a contrast experiment was conducted with the 5-point method mentioned in ISO 230-3. The result shows that measurement results of the presented method are consistent with those of 5-point method mentioned in ISO 230-3 and the mean relative deviation of the two methods is less than 15.8%, which verifies the feasibility and reliability of the presented method. The equipment is concise, portable, and easy to install. Moreover, the obtained results could be used for thermal error compensation of spindles to improve the machining precision of 5-axis machine tools.