The dynamics of solvent and rotational relaxation of coumarin 153 (C-153) has been studied in neat N,N,N-trimethyl-N-propyl ammonium bis(trifluoromethanesulfonyl) imide ([N-3111] [Tf2N]) and its mixtures with polar solvents, namely, methanol and acetonitrile at three different temperatures from 294 to 303 K. Both the solvent and rotational relaxation dynamics of C-153 in neat [N-3111][Tf2N] are linearly correlated with the bulk viscosity at different temperatures. The solvent relaxation time and rotational relaxation time of C-153 decrease with gradual addition of cosolvents in [N-3111][Tf2N]. The gradual addition of cosolvent decreases the viscosity of the medium, and consequently, the solvation and rotational relaxation time also decrease. The decrease of solvation time is more pronounced on addition of acetonitrile compared to methanol.