Abstract: A kinematic-parameter calibration method for the reconfigurable modular robot system is studied based on the local product of exponential (local POE) formula, and a calibration method for modular robot based on sub-assemblies is presented. Firstly, the sub-assemblies are described in the twist exponential form, and their kinematic model is established. Then, an actual kinematic model for modular robot based on sub-assemblies is established by using the local POE formula, which includes the joint constraint conditions. By differentiating the kinematic model and employing the explicit expression of the differential of the exponential mapping, the explicit expression of the mapping relationship between the position errors at the end of the modular robot and the errors in the joint twists of the sub-assemblies and in the local position of the terminal sub-assembly is given. Finally, a modular reconfigurable robot system is taken as the experiment platform, and some tests are carried out with the help of a laser tracker. The test result shows that the calibration process can converge to a stable value. After the parameter calibration, the mean value of the modulus of the positioning errors of the 6-DOF (degree-of-freedom) modular manipulator in the tests, is reduced by about 95% and the maximum value is reduced by about 92%.