Weak localization modulated by gate voltage in bilayer graphene was studied experimentally. A transition from weak localization [near the carrier charge neutrality point (CNP)] to weak antilocalization (away from the CNP) was found. The suppressed intervalley scattering due to screening of atomically sharp defects and spin-orbit coupling regulated by gate voltage can explain the experimental results well. Our experimental results confirm the theoretical prediction that the weak localization in bilayer graphene is strongly suppressed by the trigonal warping and it is only present in systems with pronounced intervalley scattering. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3505310]