Thermally activated delayed fluorescence (TADF) is fluorescence arising from a reverse intersystem crossing (RISC) from the lowest triplet (T-1) to the singlet excited state (S-1), where these states are separated by a small energy gap (Delta E-st), followed by a radiative transition to the ground state (S-0). Rate constants relating TADF processes in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) were determined at four different solvent polarities (toluene, dichloromethane, ethanol, and acetonitrile). We revealed that the rate constant of RISC, k(RISC), which is the most important factor for TADF, was significantly enhanced by a reduced Delta E-st, in more polar solvents. The smaller Delta E-st was mainly attributable to a stabilization of the S-1 state. This stabilization also induced a Stokes shift in fluorescence through a relatively large change of the dipole moment between S-1 and S-0 states (17 D). Despite of this factor, we observed a negative correlation between Delta E-st and efficiency of the delayed fluorescence (phi(d)). This was ascribed to a lower intersystem crossing rate, k(ISC), and increase nonradiative decay from S-1, k(nt)(s), in polar solvents.