By using reflection high-energy electron diffraction (RHEED) and RHEED multislice dynamical calculations, the atomic structures of the (root3 x root3)R30 degrees reconstruction on 6H-SiC(0001) surface were solved. Both the simple adatom structure with a Si coverage of one-third monolayer occupying the threefold-symmetric T-4 or H-3 sites and a bit complex structure with Si trimers centered on the T-4 positions with 1 monolayer coverage are all compatible with our results.