A relativistic ab initio model potential (AIMP) method with the third-order Douglas-Kroll (DK3) approximation has been developed for the whole series of the actinide elements from Th to Lr. Two different cores, i.e., [Xe, 4f,5d] and [Xe, 4f], have been employed and the corresponding valence basis sets, (14s10p11d9f)/[6s5p5d4f] and (14s10p12d9f)/[6s5p6d4f], are presented for all actinides. The mean absolute errors of the AIMP relative to the all-electron results for the atomic SCF valence orbital energies (epsilon) and the radial expectation values () are 0.003 (0.001) hartree and 0.004 (0.006) bohr with the small (large) core set. The spectroscopic properties of the (1)Sigma(+) ground state of thorium monoxide, ThO, are calculated at the SCF and complete active space SCF levels. The DK3-AIMP results again satisfactorily reproduce the all-electron DK3 results. The large core set gives almost the same results as the small set for atomic and molecular calculations, suggesting that the 5d electrons can safely be omitted from the valence electrons in actinide chemistry.