Time-resolved observations of brown dwarfs rotational modulations provide powerful insights into the properties of condensate clouds in ultra-cool atmospheres. Multi-wavelength light curves reveal cloud vertical structures, condensate particle sizes, and cloud morphology, which directly constrain condensate cloud and atmospheric circulation models. We report results from Hubble Space Telescope/Wide Field Camera 3 near-infrared G141 taken in six consecutive orbits observations of HN Peg B, an L/T transition brown dwarf companion to a G0V type star. The best-fit sine wave to the $1.1-1.7mu$m broadband light curve has the amplitude of $1.206pm0.025%$ and period of $15.4pm0.5$ hr. The modulation amplitude has no detectable wavelength dependence except in the 1.4 $mu$m water absorption band, indicating that the characteristic condensate particle sizes are large ($>1mu$m). We detect significantly ($4.4sigma$) lower modulation amplitude in the 1.4$mu$m water absorption band, and find that HN Peg Bs spectral modulation resembles those of early T type brown dwarfs. We also describe a new empirical interpolation method to remove spectral contamination from the bright host star. This method may be applied in other high-contrast time-resolved observations with WFC3.
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