Flavour Physics Results from ATLAS

We present here a selection of B-physics results from the ATLAS experiment taking data at the Large Hadron Collider (LHC). The ATLAS data collected in $pp$ collisions during $2010$ and $2011$ allowed a good quality of B-physics measurements, reproducing essential B-hadron properties, such as masses and lifetimes, and demonstrating a good performance of the detector within an increasing instantaneous luminosity of the LHC machine. These features enabled in particular the search for rare B decays with a precision that contributes to the LHC potential. The search for the decay $B^0_s o mu^+mu^-$ using $2.4$~fb$^{-1}$ of data collected in the first half of $2011$ is described. The observed number of events is in agreement with the background expectation. The resulting upper limit on the branching fraction is ${ m{BR}}(B^0_s o mu^+mu^-) < 2.2 imes 10^{-8}$ at $95%$ confidence level.