In this work, we exploited time-resolved fluorescence polarization anisotropy (TRFA) to characterize un-doped and doped carbon dots (CDs). The rotational correlation time related to the size of the particle through classical Stokes-Einstein -Debye equation. The TRFA technique applied in this study achieves picoseconds time resolution, which approximately corresponds to the particle size determination at sub-nanometer precision. The calculated diameter of the CDs from the measured depolarization time constants in aqueous solution is well matches with the actual size of the CDs within the precision. This study proves that the TRFA method is highly complementary with other size determination techniques.