A new type of coplanar transmission lines was presented, including step microshield coplanar waveguide (SMCPW), step microshield coplanar coupled line (SMCCL), and coplanar coupled step microshield line (CCSML), which are suitable for MMIC design. The simple, analytical and practical formula for the evaluation of the quasi TEM characteristic parameters was derived, which is based on a conformal mapping procedure. The numerical calculation results show the characteristics of these coplanas transmission lines.