Thermalisation and possibility to use statistical mechanical description, is considered to be a feature of ergodic dynamical systems, which can therefore be treated as stochastic systems. We show that the same result can be achieved also for integrable systems, which remain completely deterministic over the course of their evolution. We show that macroscopic thermodynamic behavior is revealed without the notion of ensemble and the ensemble averages, and uses instead the time averages of microscopic variables. In particular, we derive the first law of thermodynamics for integrable systems and the second law of thermodynamics for their “equilibrium” evolution. Furthermore, we provide the expression for the deterministic entropy. The analysis utilizes the property of adiabatic invariance, a unique feature of integrable dynamic systems.