In the present study, rotational relaxation time in N2-N2 system is predicted based on a rotational state-resolved master equation analysis in isothermal initial heat bath conditions. Required rotational state-to-state cross section is obtained by an atomistic quasi-classical trajectory method. To reduce computational cost drastically, the rotational and vibrational states are separated, and the vibrational states are assumed to be in a Boltzmann distribution. The present rotational relaxation time, determined from the state-resolved master equation analysis for the temperatures of 10,000 K, 20,000 K, and 32,000 K, is discernibly different with the existing data. The postshock nonequilibrium radiation prediction is then made to apply the present rotational relaxation time to a typical Earth reentry condition, and the calculated intensity profiles are in good agreement with the shock tube experimental data.