We develop a novel method to measure six geometric errors (two position-independent geometric errors and four position-dependent geometric errors) of a rotary manipulator; we use two displacement sensors and one measurement target to this end. To enhance measurement accuracy, the method initially removes setup errors that distort the sensor data and then models the geometric errors using an nth-order polynomial equation exhibiting C1-continuity. The data are used to define the relative positions of the reference and measurement co-ordinate systems using a homogeneous transform matrix. The geometric errors of a rotary manipulator are measured using the least squares method to exploit the linear relationship between the errors and the measurement target profile. We validate our method via simulation and assess measuring uncertainties with consideration of measurement noise.