Relation among structure of Ru underlayer and atomic layer stacking structure and uniaxial magnetocrystalline anisotropy (K_u) of sputtered Co films was discussed. Small torque with amplitude below theoretical limit |K_u^Cobulk-2π(M_s^Cobulk)²| = 6.8×10⁶ erg/cm³ was obtained for Co film with thickness d_Co ＜ 30 nm deposited on rough Ru underlayer. Structure analysis of the thin Co films with d_Co ＜ 30 nm deposited on rough Ru underlayer demonstrated that: 1) Magnitude of stacking faults were less than 1%, which meant the Co films had almost perfect hcp stacking, 2) Lattice constant c was expanded about 0.6% with retaining lattice constant a, 3) Thin Co films had rough surface which reflected morphology of the Ru underlayer. According to these results, it was thought that initial Co growth reflected vertical stacking of Ru underlayer on inclined crystal plane of the rough Ru underlayer. K_u was derived by correcting torque amplitude with self-demagnetizing energy considering surface morphology of Co film. K_u took maximal value of 5.0×10⁶ erg/cm³ at d_Co = 30 nm, and rapidly decreased with decreasing d_Co. Reduction of K_u in d_Co ＜ 30 nm might be caused by decrement of crystallographic uniaxial anisotropy for Co film due to expansion of the c axis.