  We have studied the phenomenon of electromagnetically induced transparency
(EIT) of 87Rb vapor with a buffer gas in a magnetic field at room temperature.
It is found that the spectral lines caused by the velocity selective optical
pump effects get much weaker and wider when the sample cell mixed with a 5-Torr
N2 gas while the EIT signal kept almost unchanged. A weighted least-square fit
is also developed to remove the Doppler broadening completely. This spectral
method provides a way to measure the Zeeman splitting with high resolution, for
example, the {\Lambda}-type EIT resonance splits into four peaks on the D2 line
of 87Rb in the thermal 2-cm vapor cell with a magnetic field along the electric
field of the linearly polarized coupling laser. The high resolution spectrum
can be used to lock the laser to a given frequency by tuning the magnetic
field.
