Hi @bayram​,

You might want to give LAStools a try. We have a series of custom transformers in the FME Hub that wraps up LAStools for use in FME. In particular I think that LAStools.lasground to just classify bare ground may be what you're looking for, and once classified you can use a PointCloudFilter to filter for bare ground and direct that feature to your writer feature type. You can download it from the FME Hub or type the name directly into the canvas in FME to add it to your workspace.

While we don't have any tutorials on using those specific transformers, you might also want to check out our article Working LiDAR Data and FME to get some more ideas and examples on working with and filtering point clouds.