What is MI?
When you say that you add some additional parts in assembly area, do you mean that you build them in the context of the assembly or that you just add some existing parts into the assembly?
Welcome to SOLIDWORKS forums Rahul.
Are you using the mass properties option to get the MI (Moment of inertia). Can you attach some picture of what kind of issues you're seeing.
You might also find this post helpful for your future reference.