Time-resolved Faraday rotation spectroscopy is used to study the spin coherence in colloidal CdSe quantum dots at room temperature. Spin dephasing time and relevant dephasing mechanisms are analyzed in different transverse magnetic fields. The exciton spin-dephasing time is 102 ps in a zero magnetic field, which is affected by hyperfine interaction between electron and nuclear spins. In a transverse magnetic field of 250 mT, the exciton spin-dephasing time becomes 294 ps due to the fact that the presence of magnetic field makes the nuclear spin fluctuations unimportant. Further increasing the external magnetic field, the spin dephasing time becomes shorter. The magnetic field dependence of the exciton spin dynamics shows that the spin dynamics is dominated by the inhomogeneous dephasing in high magnetic fields (≥ 250 mT).