Recent advances of the interventional MRI, ‘surgery in the Open MRI’, proceeds safer and assured minimally invasive therapy with a plenty of useful information of MR images acquired during the operation. In this paper, we present a small MRI compatible robots for horizontal gap open MRI which controls needle orientation effectively in narrow gantry space. Registration between MRI, MR image, and the robot is performed with affine matrix transformation using 100 small markers and the 3D measurement sensor. Markers are distributed with 20mm clearance and each 50 (half of 100) markers represent the XZ- and YZ- plane. They are mutually-perpendicular to achieve accurate registration. The accuracy in the total system was 1.24 to 4.10(mm) including the registration error and the mechanical error and MRI image distortion.