I have simulated in the same way: first in ADS and then in CST. The results on CST depends on the port. If you are using a microstrip line, you can use waveguide port.

Thanks for the reply luis,  If I use wave guide port the sparameter results are worst when compared to Discrete port.

Port creation : I selected the face of the microstrip adjusted the length width as per CST literature. Any other  thing to take care while using wavegudie port? because i doubt my wave port is correct?

When you use waveguide ports they shoul be large enough. For the port dimension you can use  

Home->Macros->Solver->Ports->Calculate port extension coefficient.

Another  thing that you have to see if the orintation vector is derected into the structure.

Port->Orientation (you can choose positive or negative) On the model you will see if the vector is directed into the model or not 

My Suggestion is

1.Check the port Impedance(Waveguide port) is 50 ohm.

2.we cannot get the same result as ADS because ADS will be simulating  in infinite ground environment, so give extra 10mm on both side of the substrate (In CST Simulation). This will reach some more accurate result not exact.

3. Give 3 mesh lines along the width of the strip line.

Discreate Port: If you know 50ohm is impedance of the designed microstrip line then go ahead with Discrete port.....If you don't Know then

Waveguide Port: If you don't know the impedance of the line,  assign the waveguide port with increased length (K*width)