Mobility for a great portion of robot mechanisms having over-constraint and non-holonomic constraints has not been clearly identified. This work us to introduce a method of mobility analysis for such systems using the concept of representative screws and pseudo-joint. The pseudo-joint is employed to effectively represent the real motion trajectory due to the rolling contact of the wheel. To show the validity and effectiveness of the proposed method, mobility of various types of planar mobile robots having over-constraint and non-holonomic constrains are examined.