THERMOSIM is a program to quantitatively predict the thermal conductivity by phonon vibration. In the simulation cell, one can set regions at which the temperatures are controlled constant. By measuring the transfer of thermal energy due to a temperature gradient in the simulation cell, thermal conductivity by phonon vibration can be quantitatively evaluated. Various types of potential functions are available in the program. This program has been applied to the studies of various systems such as lubricants, solid crystals, etc.