Laser Doppler velocimetry is used to measure the velocity profile of turbulent thermal convection in an aspect-ratio-one cell filled with water. Velocity fluctuations are found to be homogeneous and isotropic in the turbulent bulk region. Despite the large velocity fluctuations, the mean flow field maintains a large-scale structure, which rotates and oscillates in a coherent manner. The experiment suggests a unique driving mechanism for the large-scale coherent rotation and oscillation in turbulent convection.