Cheng et al. gave the microscopic expression of entransy in the ideal gas system for monatomic molecules in an approximate non-interaction particle system. Diatomic molecular gas or polyatomic molecular gas is more widely used than monatomic molecules gas in industry. So this paper deduces the microscopic expression of entransy in the ideal gas system for diatomic molecules by using the approximate calculation of partition function. (C) 2018 Published by Elsevier Ltd.