What do you have as an initial file? Any pdb or something like that to use?

Dear Marcel, I am doing fatigue modeling. so i need  polycrystal structure as lmp file format. so, this file i will load in my input file through command read_data. Do you know any software package?.

Might help:

https://github.com/jdevemy/lammps-tools

Thank you Marcel, but this is not for creating polycrystal.

It can be utilized to create polycrystals.

Also check this one:

http://li.mit.edu/Archive/Graphics/A/utils.html#voronoirize

In any way, you would need to modify things for yourself.

Thank you Marcel.