The relationship between uniaxial magnetic anisotropy Ku and lattice deformation was investigated for Co-Pt and Co perpendicular films and compared with theoretical results. A study of Co100-x Ptx Ru perpendicular films (x=10,14,20) of various thickness showed that Ku increased as the ca ratio of the hcp Co-Pt lattice decreased, and the increase of Ku for a 1% reduction of ca was (8-10) × 106 erg cm3. The ca reduction was accompanied by an increase of the lattice volume Vlattice of about 2%. An examination of Co75 Pt25 films deposited on various seed layer materials showed almost the same relationship between the value of Ku, the ca ratio, and Vlattice. Moreover, the rate of increase of Ku on reducing ca for single crystal Co perpendicular films was nearly the same as for the Co-Pt films. A theoretical calculation of Ku based on the linear muffin-tin orbital method in the atomic-sphere approximation revealed that both a decrease in the ca ratio and an increase in Vlattice enhance the Ku value of hcp-Co. The theoretical evaluation was qualitatively in good agreement with the experimental results, although the rate of increase of Ku evaluated experimentally was three to four times larger than the theoretical calculation.