  We investigate the formation of methane line at 2.3 ${\rm \mu m}$ in Brown
Dwarf Gliese 229B. Two sets of model parameters with (a) $T_{\rm eff}=940 $K
and $\log (g) =5.0$, (b) $T_{\rm eff}=1030 $K and $ \log(g)=5.5$ are adopted
both of which provide excellent fit for the synthetic continuum spectra with
the observed flux at a wide range of wavelengths. In the absence of
observational data for individual molecular lines, we set the additional
parameters that are needed in order to model the individual lines by fitting
the calculated flux with the observed flux at the continuum. A significant
difference in the amount of flux at the core of the line is found with the two
different models although the flux at the ontinuum remains the same. Hence, we
show that if spectroscopic observation at $2.3{\rm \mu m}$ with a resolution as
high as $R \simeq 200,000$ is possible then a much better constraint on the
surface gravity and on the metallicity of the object could be obtained by
fitting the theoretical model of individual molecular line with the observed
data.
