A robotic 3D printing system has a six degree of freedom (DOF) robot that holds the platform on which the 3D part is built on. The system uses the dexterity of the 6 DOF robot to move and rotate the platform relative to the 3D printing head, which deposits the material on the platform. The system allows the part build in 3D directly with a simple printing head and depositing the material along the gravity direction. The 3D printing head can be fixed relative to robot base, or moved in the X-Y plane with 2 or 3 DOF, or held by another robot or robots. The robot movement can be calibrated to improve the accuracy and efficiency for high precision 3D part printing.