WW cross sections and W branching fractions in L3

The measurements of W-pair cross sections performed by the L3 experiment at the LEP collider are herein reported. The statistics collected in the period 1996-2000 amounts to more than 8000 events distributed over a large center-of-mass energy range: 161-208 GeV. From the cross section measurements, also the W branching fractions in leptom and hadrons are derived with a percent precision: BR(W to qq) = 68.47 +or- 0.49 +or- 0.33% and BR(W to lv) = 10.51 +or- 0.16 +or- 0.11%. The improving statistical accuracy of the measurements is now approaching the systematic uncertainties. Further studies, necessary also for the W mass determination, will be needed in the near future to keep them under control. (10 refs).