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