Main features of the CMS detector relevant for B-physics studies are descri
bed. A detailed simulation study of the channel B-s(0) --> mu(+)mu(-) is pr
esented. Results for some rare FCNC B-d,s(0) decay modes and sensitivity to
the forbitten tau --> 3 mu decay are also discussed. Expected CMS reach fo
r the x(s) measurement in the B-s(0) - (B) over bar(s)(0) oscillations is a
lso shown.