@xuKyosuke
The most likely cause for jitter is that you are trying to set the position on all networked systems when you should only be setting the position on owners, or the server if there is no owner. Smooth Sync will handle all position changing on non-owners (or clients if there is no owner).
Smooth Sync will work with each of the templates provided by Unreal Engine as well. So it may be beneficial to see how those work and then maybe you can see what the difference between your project and the templates provided by Unreal are.