LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) is the most used Open Source Software (OSS) to study mechanial properties with molecular dynamics simulations in the field of polymer, carbon and metal materials. The LAMMPS is developed and maintained by Sandia National Laboratory, USA. The author considers the advantages of LAMMPS for beginners to be the simple input-output files with text format, seamless from single PC computing to massively parallel computing and easiness of modification of program codes. The LAMMPS is also widely used by Japanese researchers. The author gives an introductions for beginners and showcases results and examples by several Japanese researchers.